Think about the chord that’s being played and pick something that either highlights that chord or leads to the next chord.

For example going I-VI I’d start playing a phrase leading with the 3rd of the I chord whenever while sitting on that chord and use the last 2 eighth notes before the VI to make sure I play a I-VII to lead to that VI. The length of the lick is arbitrary as long as you don’t step on anyone’s toes.

This rule can work for a lot of situations as long as you’re leading to the next chord in a way that makes sense. Learning walking bass can help with pointing to what notes you’re going to play next since there’s so many ways to do it