the whole "sing it first" thing comes from the concept of "hearing" your idea in your head before you grab the guitar, the singing makes the idea concrete and then your melody could be transfered to any instrument as long as it works with the key of the song.
what are the chords in your progression? (this helps us determine the "key")
if your not sure please try to tab them out for us... depending on the chords i could give you quite a few guitar ideas/techniques that will help you put together a solo
thx