Tokøta from Mastering.com walks through Logic's step sequencer in melodic mode, showing how to program a pitched bassline instead of just triggering sounds or samples.
Switching a track to notes opens up a full grid of pitches, but by default you only get one octave to work with. Loading a pattern template (minor, then swapped to major) expands that to two octaves so there's more room to write an actual line.
To move notes into the right octave, use Shift-Option-Arrow the same way you would in the piano roll, or use the dedicated octave tool for a more visual approach. Both get you to a simple, droning bassline that locks in with the rest of the groove.