Depending on where you want to start.
Basic set up: Put a ball on intersection of the first diamond short rail and first diamond long rail from corner pocket. Put another ball at the intersection on the opposite short rail corner pocket. Start with ball in hand. Make first ball into its corner, then shoot other ball into it's associated corner. pocket. Keep spotting balls once they are made. See how long you can keep going before missing.
Level 2: Keep cue ball below the 3rd diamond up.
Level 3: force the cue ball off a rail after each shot, again keeping cue ball below 3 diamond (of half table if you want)
Level 4: place 2 more balls on the other end of table at same intersections. Shoot at corner each in succession clockwise or counterclockwise. Again Spotting each ball after pocketing. Again see how long you go around without missing.
Level 5: Same as level 4 but zig zag from end to end. This forces longer table follows for the cue ball.
Optional - put a 3rd and/or 4th ball on head spot and foot spot. Make them the last balls in the rotation again spotting them.
2nd order of difficulty: Start with the setup of each balls at intersection of 2 diamonds along long rail, one diamond on short rail. Repeat levels 1-5.
Again, each ball must be made into its associated pocket.
You could mix up by putting one or two balls on the rails. To make it more challenging. randomly add dead balls into the mix that you have to play around (you cannot hit them after the object ball is pocketed with the cue ball).
Another exercise comes from Tor Lowery in this video. (not really a drill, but a good way to challenge yourself to see how you are improving.)
https://www.youtube.com/watch?v=AgdrAwaaCBU&t=536s