How can go about writing a chord progression in a specific key, then change chords chromatically, and then finally end up back the to original key.  For example:  Key: C Major  Song starts in C  Then in the bridge, it changes to different chords that are not the C Major scale.  But then finally, at the end of the bridge, the song goes back to C Major.
you'll have to play through the entire song up until that point and try every chord you know after that and see which one you like