Having trouble writing MUSIC for songs (lyric, guitarist, solo, artist)

[Astronomer]

Hey everyone, I noticed in this topic there is countless numbers of threads about writing songs in the lyrical sense, but hardly any about writing songs in the musical sense - so I hope I'm posting in the right area but anyway here goes...

I was just wondering how you guys go about writing MUSIC for songs. What kind of creative process do you follow?

I've recently joined up to a new band and am finding it difficult to create music. I've never had this problem before as a solo artist and in previous bands, but I'm just finding it hard to create stuff musically (not lyrically) and collaborate with others. I play piano, and in particular I find it difficult to create stuff with my guitarist.

So yeah, just general how you go about creating new music, new sounds. Creative processes you follow, etc. Do you just mess around until something sounds good? Do you follow a particular process? Any ideas would be great!