How to Play Bass and Melody at the Same Time
Playing bass and melody together sounds like it requires doing two things at once, but the real secret is the opposite - each part needs to stop requiring attention on its own before it's even worth combining them.
Two voices, not one motion
The bass (usually the thumb) and the melody (usually the index, middle, and ring fingers) are really two independent musical voices played by one hand at the same time, not a single complex motion. Treating them as two separate parts rather than one trick immediately simplifies practicing them.
Play each voice alone until automatic
Play just the bass part all the way through the phrase until it's fully automatic. Then do the same with the melody - on its own, with no bass. Combining the voices while either one still needs conscious attention means training confusion, not coordination.
Combine one bar at a time, not the whole phrase at once
Take just the first bar of both parts and combine that one before moving to the next. Trying to play the whole phrase with bass and melody together right away spreads attention too thin - within one bar it's far easier to hear exactly where something comes apart.
Listen to each voice separately while playing both
While playing them together, try deliberately shifting your attention: listen only to the bass, ignoring the melody, then the other way around. This isn't the same as just playing both parts - deliberately listening to one voice helps you notice exactly where it loses stability under the other's influence.
A slow tempo isn't a temporary crutch
Combining bass and melody almost always needs to start noticeably slower than either part was played on its own - the combination itself adds difficulty, regardless of how confident each part sounded alone. Raise the tempo only once the combined version is stable at the slow one.
Try it in practice
Open the metronome