I've been teaching myself metal bass and sometimes I practice with my drummer friend. I just improvise with familiar scales and patterns, my drummer follows my lead and sometimes we get into a decent groove, other times I get a block and resort to playing songs I've been learning. I enjoy it and learn from it regardless but we never seem to get anything written down. On a few occasions a guitarist friend has joined us and it was far worse. The drumming was ok and the guitarist didn't seem to struggle playing along to my bass lines either, I couldn't tell him exactly what I was doing but I could show him and he'd figure it out himself. I struggled to play along with the riffs he'd written though, I tried watching and listening to what notes he was playing and working out where to play mine but I found this overwhelming when put on the spot. If what he was playing was written down in front of me that would really help but I need advice on how exactly we should be practicing and writing together.