1 - I usually start by wanting to play a little bit over a simple drum/snare pattern. Not that I'll use them but horrible timing and stuff, without a basic drum pattern.
2 - Build a loop of maybe 16 bars. Maybe add some variations.
3 - Realize I'm now stuck in a loop that I've been messing with for hours. Turn the project off.
4 - Come back later, deactivate or delete stuff and play something else, possibly add new instruments.
5 - Realize I'm still stuck in a damn loop...Leave again.
6 - Come back again. Now hopefully have multiple different midi clips to pull variations from. Build a bigger loop.
7 - Never come back again.
8 - The end.
PS - Other than the joking bits. This is how I usually start projects. By building layers into a loop. Hopefully by building enough variations and instruments to get time laid down. If I get stuck, I just come back later and deactivate some stuff and play some more. Speed up or slow down my drums, etc.
See where it gets me. If that doesn't work, I'll start projects with failed bits from other projects with some drums and start playing again.