most of the time the way it works for me is that i go in with a certain sound or genre in mind and then just roughly patch something together and then just keep developing it until im satisfied with the end product. other times i just fuck around until i get something i like without much plan or anything.
for example, this song was entirely one shot and completed in less than an hour:

while the one in the post above took a few days (maybe 2-3 sessions off and on) to finish.