For me it all starts with words.
 
Don't need SONAR for that.

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.
All this is recorded of course if not in Sonar then at least to my Zoom H4.
If not yet in Sonar, now's the time. Doing all this in Sonar let's me cut and paste song sections which at different stages of development could be anywhere on the time line.
I continue to complete the arrangement of verse intros, chorus etc with acoustic guitar and additionally record any lyric phrases that happen along the way.

I collect all the hooks both musical and lyrical that are recorded then arrange them appropriately till the song and lyric content is defined.
Then I clean up the structure which gives me all my song parts, intro, verse, chorus breaks etc. Once I have that locked in I begin recording the instruments I want.

The acoustic guitar rhythm will suggest which midi loop I use as drum accompaniment through Sup Drummer rather than the metronome .

I start fleshing out the drums for bridges, fills, buildups etc and getting the kick and bass line in the right place along with general dynamics.
I'll replace the acoustic guitar now if it's not staying and make decisions about the instrumentation at this point. Once all the parts have been recorded, it's time for polishing the arrangement as it will affect any mixing.
Only once I have the arrangement down pat will I start mixing.

For me, drums are it. It is where I get inspiration from to create the rest. Drums followed by basslines then chords then lyrics then melody.
 
BUT...I am HOPELESS at drum programming (even with all the drum patterns with SD and AD) so I don't really write that much, as it takes forever just to decide on a drum pattern/s with the result that I get frustrated and end up giving up and doing the gardening!. I would love to write more.

The question asks how we write but the answer seems to be how to track.  Maybe I am misreading it.
 
• I always start with an idea and I can't explain how that happens.  It is usually a tune or partial tune with a hook line.
• Then I develop the melody.
• Then I outline the song with a phrase or sentence to explain the idea(s) I want in each verse and chorus.
• With that done, I start to craft the actual lyrics, which may cause me to rearrange or change the outline as I go.
• In tracking, I usually do rhythm acoustic guitar (or piano if that is the main driver); a track (like electric piano) that plays the melody and has the lyrics; bass; drums; and keyboards in that order.  Next I overdub additional rhythm instruments and sweetening (strings and orchestra).  Last is fills and leads, if any.  Of course, I may jump around as needed and of course I go back and revise; however, it is important to me to establish the feel before doing too much else.
• Vocals and harmonies come after the track is complete.  Some people do scratch vocals, but I use the melody/instrumental track for that.  In my style of writing, there is no much vocal ad libbing.
 
 
 

I create a "Dummy" midi track that triggers a piano sound outside of Sonar (my piano midi controller)  .  I view this track in Staff View.  I will start by improvising a melody on the dummy track. In piano view I mess with melody choices and in Staff View I add chords and sometimes lyrics.  When the song is ready I start orchestrating using the dummy track staff view as a guide.   

1. Get an idea, usually a hook. 
   
2. Then, I'll start a new Sonar project, set a tempo that matches my idea/hook and lay down a chord pattern with a MIDI instrument, usually a piano, to a click track. Starting with a MIDI instrument allows me to easily adjust the project's tempo or key, if necessary, before going any further.
   
3. I record whatever lyrics I have come up with and sing nonsense syllables for places where I have melody ideas but no words yet.
   
4. Once the hook is laid out like I want it, I then slide the whole section down the timeline and begin to work on a first verse, then a second verse, then a middle eight and finally, an intro and an ending. Sometimes verses/middle eight lyrics & melody come first; other times I just lay down progressions for those parts and see where they lead me. I copy my original hook section to after the second verse and to the end, if the arrangement calls for a chorus end.
   
5. The entire arrangement is then fleshed out with additional instrumentation and vocals, sometimes totally replacing all of my original idea tracks. (As I perform steps 4 & 5, I gradually come up with final lyrics.)
   
6. Finally, I make several test mixes, listening to each in my car and on other systems. I make notes of mix inconsistencies that become apparent and perform further mixes to shoot for perfection.
   

 
I don't ALWAYS write this way, but it is the most common way I write. After laying down the original melody and MIDI progression, I have no set order in which I record drums, guitars, bass, etc. I simply go wherever the muse leads me, always with the goal of showcasing the lyrics and melody.  

Eesh I wish I had a regular process!
 
I keep a ton of sketch projects, just a part and some accompaniment of some kind. So I trawl through those regularly to see what the heck I was thinking.
 
I think I often start with a bass or synth bassline. Often times this becomes a hook and I record a second, actual, bassline. Sometimes I build something up from a production trick, like a killer sample or a weird drum part.
 
Inevitably I've got a part and some drums (which I usually disassemble and rebuild later). Next is figuring out if I've got a chorus or a verse happening, and then I write the other part. Usually adding another melody line of some sort along the way.
 
After that it's a quick drag-copy to create a basic structure. There is usually where I determine if the track is crap or not. Almost always there's a bridge or solo section to be written. This is where I remember the vocals!
 
Sometimes I write a vocal melody there, other times I build the rest of the track up and save those for last (more on that)
 
If I'm feeling it, I'll start adding ear-candy production stuff at this point, choosing final patches, re-doing the drums. If parts are critical to the song (like tempo delays or whatnot) I'll add them. 
 
Lately I've been doing a "rough mix" before vox, just to get it in place. I've gotten pretty good and carving out vocal space so I'll try and get a mix pretty far along.
 
Then, if i haven't already written a vocal line, I'll bring in my writing partner and we "go fishing" for vocal hooks. I'll usually do a verse or two lines of a chorus to have some words to mush around with that give a gist of where I think the song is going. More often than not these remain but we're both open to chucking them if something better comes up. 
 
I do some editing of the hooks to mate parts together and then we fill in any gaps until we've got our melody. Then one of us (I let her take first crack usually) goes and does a draft of lyrics, which we track in and tinker with.
 
Then I play with the mix some more and stockpile the track until we've got about twelve or so and then do final vocal tracking for a record! Though we're moving more towards working track by track. My writing is the slowest part of the process so I try and build up a supply of songs to work with before we get rolling.
 
Anyhow, final mix comes after vox, and I'm getting much faster at busting those out. Working about three hours a night it takes me three days to do a song. We use a lot of electronics and I like to take it around and listen on various systems so it takes a couple of days to really get a sense of balance.

Interesting thread.
 
I very rarely start to record anything without having a fairly complete idea in the first place. Of course, things start to develop once you do begin to put them down, but that tends to be more a matter of arrangement than writing. I almost never record snippets. I like to let things percolate in my head for a while before going anywhere near a recording process.

I still struggle with it. I end up spending so much time trying to figure out how to get around, that I lose the inspiration.

Ahh but that's why you need to have a tool for quick easy recording, even a cell phone can do this now, but a dedicated recorder is certainly a good investment for serious songwritters.
 Those little snippet ideas are what can turn into your best songs.
By the time I get up from noodling on the couch, go to the studio room, fire up all that equipment, move the mike, look for my headphones,,, I forget why I came! 
 
I used to use the kitchen getto blaster which still has a working cassette and built in mikes. 
I have been known to get out of bed at 1 AM to go write a lyric down.