On guitar, your strumming hand keeps time. On piano, the left hand usually does that job. These drills retrain that habit. Each one is a little harder than the one before.
Before each drill, tap it on a table. Your left hand taps the bottom stave, and your right hand taps the top. Count out loud: “1 and 2 and 3 and 4 and”.
The left hand holds one long note while the right hand walks.
Now swap jobs. The left hand keeps the beat while the right hand holds.
The right hand plays twice for every left-hand note.
The hands never play at the same time. The left hand plays on beats 1 and 3, and the right hand on beats 2 and 4.
The right hand plays only between the beats. This is the hardest drill for most guitarists.
This drill combines the earlier ones. The left hand plays a rolling pattern while the right hand plays a short tune.