I nearly never record a song I already have written. I don't know what it is but it is doing something new that interest me. Those songs are hammered out on my acoustic usually and I sometime perform them.
 
I love finding magic! Those accidental combinations that just spark something to life.
It is almost always starts with a bass line I am working on but not always, it could be keyboard alot of the time guitar.
I often practice scales in different keys and ideas come aplenty. It is interesting how different keys lend themselves to a different expression. I'll even play with a capo to get a new tonality or creative angle. 
That idea of playing a different instrument than you usually do also can inspire.
 
Sonar starts as a multitrack recorder for me. 
I load up AD and get a drum loop that I like.
 
I often harmonize vocally to a guitar progression and that way I know it works as a progression.
I'll get a basic arrangement happening in the key I am favoring at the time.
 
I'll get on bass and lay over it. Often the magic happens here but it could spark anywhere.
If I drop it there then no spark and start afresh.
 
If I am nutting out parts for the new arrangement then I am inspired and honing continues.,
Sometimes that inspiration takes me into mixing it. And ashamedly have mixed some tunes that were incomplete vocally.
It is rare I get a vocal done due to lack of interest in lyrics and something to sing about and confidence. I do need to remedy 
 
The progression often becomes an instrumental if I have worked on it to the point of mix/automation it will have an okay melody or hook that keeps me going at it.
 
I am reading how others are using sonar for songwriting and I am sure some ideas can be put to use to help my own productivity.
 
Already the idea that the song starts with the lyrics is correct. I have found when writing with another the lyric content determines the music and a much more melodic production can be  produced.
 

Common endpoint when recording without a real song plan before hand:
- Have recorded a nice drum part
- Have laid down a nice complex chord progression on electric guitar
= Can't go anywhere!
 
Project ends up gathering dust on my HDD for years. Maybe it's a bridge part for a song I haven't written yet?

All depends, if I am writing for myself of with someone else.
 
For myself I sit at the keyboard, sometimes with something in mind, and just see what falls out of the furry paws.
 
When I am writing with someone else I take my lead from them either from their lyrics or the melody they sing and again see what drops out of the furry paws.  Usually this method starts with an acoustic guitar to get a basic chord progression and general rhythm/feel.

More often than not?   Slowly.
 

Great thread! Interesting to see unique approaches to the same goal.

I start and then always get hung up on drums - which then kills the entire thing

I don't! I may compose them in Sonar. I may record them in Sonar. But never have I ever written them in Sonar.
 
I usually use Note pad or a real note pad.

My approach begins with a hook. It is usually a phrase. That phrase will have a rhythm to it that will dictate where it's placement in the song should be.
Longer phrases lend themselves to the last line of the A verses in an AABA format. Shorter phrases work really well in the first line in B choruses of ABABCB.
The rhythm of the hook will also point the way to the feel and tempo of the song as do the subject of the hook.
Before I even power up Sonar, I grab a piece of paper and my Martin D28 and figure what kind of song I want to come up with. I work with a lot of co-writers
in Nashville and we will write with specific pitches in mind. Once I have the skeletal parts of the song feeling good I'll record myself singing into my iPhone, type up my
lyric and subject matter ideas and send them to co-writers who add to the song foundation. Once we agree on the feel, song structure and tempo, I'll open a new session in Sonar.

Now comes the recording of the work tape.

I begin by setting the tempo and recording a reference track with me singing with my guitar to track one and place markers.
Becoming more familiar with the song allows me to program a scratch drum track either using a simple loop or sequencing a bare bones drum track using either Session Drummer or
Addictive Drums.
I'll then lay down a bass track direct by going into a Radial JDI into a Black Lion B173 into a Ashly SC50 into a Motu 896mk3 into Sonar.
After that I'll further tweak the drum/loop tracks to create a better dynamic flow.
Acoustic guitars come next. Often I'll record a D28 with an LR Baggs into a Fishman Aura into the B173 for panning hard left.
Then a different guitar, usually a Martin 0018 in open G or Nashville C drunk tuning will be recorded with a Shure P81 into the B173 and Ashly and panned hard right or vice versa.
Next is the work tape vocal recorded with a Shure KSM32 into the B173 and Ashly and tuned up in the stand alone Melodyne.
I'll export that session with an SSL bus compressor and Waves L2 on the master fader to bring the level up and create an mp3 to send to my co-writers.
I save that session (and all sessions) to a bundle and redundantly save to a internal solid state drive, an external HD and Dropbox.
Feedback is gathered, tweaks are made, publisher input, more tweaks and then the demo phase begins and comes down to a matter of putting the right
instruments in the right places to support the song and the vocals.

As I demo, 99% of the time I will create the backing tracks and hire a Nashville vocalist to sing the demo with harmonies with me producing via Skype from my studio.
 
My 'finished' demos are usually:
Hired male or female vocalist
Live drums to replace or play with my programming
My acoustic guitars and bass
Hired session guitar and keys if needed.
 
I import those files into my Sonar session and mix using Waves, ProChannel stuff, SoundToys, Slate plugs and cool freeware I picked up along the way, master and deliver.
 
For me Sonar is both a drawing pad and a program to deliver mastered content.

mudgel
Start with acoustic guitar which gives me basic rhythm, tempo and dynamics.
Hum along to get the basic melody and the tonal structure of the song.

 
This is where I start as well.  I am not a great guitar player, but can at least get it to the point where it is useful to move the song along.
 
I typically do the Bass Guitar after the first rhythm and then maybe a scratch lead vocal.
 
Next comes the drums...Since I am a drummer and that is my main instrument, I have never used loops preferring the live human feel.  Although I have to admit the computer is getting closer to live drums all the time.
 
If I feel it is worth it, I will get a better guitar player than me to laydown some tracks based on the original.
Then I will track additional guitars (Both Rhythm & Lead).
 
I will use my MIDI guitar to lay down any keys/strings, etc.


Once I have a fairly nice mix, I will lay down permanent lead vocals followed by backing vocals.
 
I don't write a lot like some of you, but I have written some.  Wish I had more time to devote to this.
 
Graham Cochrane over at RecordingRevolution.com did a Song a month challenge back in 2011.
[link=http://therecordingrevolution.com/category/one-song-one-month-challenge/]http://therecordingrevolu...g-one-month-challenge/[/link]
While it is no longer active you can look at this for inspiration.
 
 
 

It all begins from a feeling that I could write something new. I grab my bass, guitar or play with Hammond or piano to improvise for a while. Usually some melody starts to get more attention in my head and it will become the seed of the new song.
 
Now things are evolving fast. I make the chord progression in minutes and make a quick demo in SONAR. First track may is usually bass or rhythm guitar. I don’t need to make any drum track – I’m used to play over the ¼ metronome.
 
After making a rough structure of the song I play electric demo drums - usually some real drumkit setu from Native Instruments Ultimate library -  and pay more attention to the groove. Now I play all instruments again 1-3 times. Each time I learn more about the song and finally I know exactly what all instruments are doing (and WHY). Mixing process has already started – it’s integral part of the arrangement and sound choices.
 
It’s time for the vocals. Demo vocals are recorded quickly but refining continues all the time to the last minute before mixing.
 
Now the professional drummer comes to studio. I give him the song structure, play the demo for a couple of times and he makes his own markings. Drums are recorded in 1-5 takes and in two hours I have got superb drum track (no editing required), including sound check and rough drum mix. I have learned that it is cheaper and faster to hire a seasoned professional than trying to make drums on my own.
 
Drums are ready. Next I play bass and guitar again and make them groove with drums. Vocals, keyboards, synth programming, rough mixing, backing vocals etc. follow in random order.
 
Mixing phase: sometimes I tend to use “advanced mixing techniques” which means that I could re-record any instrument track or even lead vocals if I have a feeling that song does not feel right. “Song is ready when it is ready”. Normally, however, mixing is just mixing.
 
Finally it’s time to send song for the mastering engineer. It’s good to have a deadline; otherwise the mix would not become ready until year 2080…