Reamping guitar: Why am I getting screetching noise/feedback? Wrong positioning of amp?

How are you connecting things?  What is the order in the audio path?
If you're getting feedback it's because you're sending the mic signal back to the amp,  you shouldn't be doing that while re-amping (or ever, really!).
 
EASY way to re-amp.  (ignoring inserts)
Record guitar dry with DI direct to Sonar. You can have any plugin you like on the track while recording so you're monitoring an "amped" sound, turn it off when done.
Set the output of this track to go a spare soundcard output ONLY,  NOT the Master bus or Main audio outputs.
Connect the spare soundcard output to your amp using your preferred method, (re-amp box, etc)
Mic the amp using your preferred method as if you were recording a live guitar.
Connect the mic to a NEW track in Sonar, set the output to your 'guitars bus' (if you use one) or the Master bus.
DO NOT send it to the same output you already sent the dry track to (ie, back to the amp)