For me, once I get a drum beat and know what notes the guitarist is playing I just sorta noodle around on the scale or arpeggio until I get something that sounds good. Maybe I'll spend some time refining it on my own.
What about you guys? Is there a certain process you follow?
I don't really think with scales, I think of what would work well to write over, trying to create a starting platform for the rest of whatever song. I almost exclusively create baselines when I songwrite tho