#1
Writing a lead part.

#1 record your rythme part
#2 listen to your recording as if its a major band playing it
#3 imagine what the lead would sound like
#4 record it by singing it
#5 listen to the recording and figure out the notes

I'll answer any questions you have. Enjoy!
#3
that method is so old, but it does work
My Music
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#4
Personally, I prefer pissing* about on a scale, recording that and picking out any good bits, then adding the rhythm guitar after. Which means nearly everything I write at the moment is in the blues minor scale, as that's the only one I know which chords are suitable for...
Having said that, don't take my advice, I have loads of lyrics and no tune that fits them and vice versa :/

*Are we allowed to say piss? If not, I mean "messing about"
#5
there shouldn't be a technical process to anything other than the recording. writing great stuff should be spontaneous and inspired, even when you cant say where the inspiration came from.