I tend to start playing and work from there. There's a first impulse that sparks the rest of it and the other layers and instruments get inspired by the first riff usually
as mentioned previously - work with stuff that comes naturally. Ease into the grooves or work against them - whatever you feel. You just have to feel it. Best case scenario - you will ALWAYS know what the next thing needs to be. You'll hear it in your head. Maybe you'll get the feeling it needs to be more like *STARTS MAKING WEIRD NOISES* and *CONTINUES MAKING WEIRD NOISES* - which is always funny, but a little demanding when you're trying to write with someone else
Also - if you have capable musicians around you, try creating basic ideas and have them finish them

Let them put character into them, creating just guidelines you feel are needed? Maybe?
Dynamics - in a band situation, I usually think that's not up to the composer anymore, it's up to the whole ensemble. It's okay to have something prepared, but from my experience, when you play the stuff with other people, it's going to change. The dynamics is the main thing that usually changes - you'll find out it can easily shift really far away from the original idea very naturally and still sound absolutely great

so my advice on dynamics is "work with the other people and feel it out"
the wankery thing - when I played guitar in a band or in my project at home, the key thing was always PHRASING. If you have great phrasing, it's really hard to be wankerous. That especially felt important in my home project which was instrumental and there was usually lead guitar - but not in the traditional guitar virtuoso way I hope

Phrasing is the key because it's connected to vocal qualities and silence.
Scales - never used scales deliberately, on purpose. I don't worry about theory except for rhythms

I try not to let it tie me up
my experiences in music are wildly different:
a) used to be in a super-experimental prog-rock band (guitar) that in the end didn't make much sense (ever). Loads of notes, long songs, lots of composition, very busy, hard to play, demanding on concentration and discipline. People would compose whole tracks (well... mainly me and the other guitar player) and bring them to the others to be rehearsed. Not much space for personality or reinterpretation in my opinion...
When you actually use that approach for something meaningful such as a conceptual album or something like that, it's amazing, it serves a purpose. We did do that as well and in the four years I spent with the band, THAT was the time when I was really happy. Otherwise it felt very dehumanized, binding, impersonal ans stressful
b) one band i play with now (bass). Joined a seasoned dude who composed and programmed his own album, now i'm in the live band. Absolutely different approach, much more songy, much more time to enjoy myself, huge space for sound experimentation... Even though it's more simple, I feel much more happier, I feel I can finally experiment more and I feel appreciated. And I'm actually usually just playing the bass lines from the album - just phrasing them a bit differently here and there, working with the timbre, dynamics and personality. We generally took great liberties with the songs - the album will be very electronic-y and live it's just a rock trio doing weird trip-hop/alternative shit with noisy guitars and rumbling basses... So we had structures, but worked with the dynamics and timbres as we felt necessary.
c) the other band i'm with now (bass). Dudes in early 20s, playing some cool rock music. Driving bass, good, solid grooves, riffs. It doesn't matter if we compose the song beforehand or if we come up with just one riff - we work on it and jam at rehearsals. If there's a whole song, the one who wrote it tells/shows the other ones the first section and we just start playing it on repeat, finding the right bass groove, the right phrasing, the right drum beat. On to the next one and the next one... then play the whole thing a few times, get used to the changes, maybe stick something in the middle, do a break, silence, sharp part, whatever it needs. Or we just try and figure out new parts as we go on. We work the songs together and just absolutely follow our hearts. The main thing is that this way, the songs are never really done. They can always be altered, revisited, reworked. They're always sort of fluid. But this is probably the most natural and casual way of doing it i've experienced so far
