Drills with two balls
The simplest drills involve only the cue ball and an object ball. With the balls in position, the shot is made and the results observed. The balls are set up again and the shot repeated. These types of drills focus on pocketing skills.
The complex drills involve the cue ball and several object balls (from two to all 15). You have to make the first ball and get into position on the second ball. If more balls are part of the drill, you have to shoot the second ball in and get shape on the third ball, and so on.
For the pocketing drills (object ball and cue ball), start with positions within your comfort zone. For distance drills, as you pocket the object ball, increase the distance in half diamond increments. Make the ball increase the distance, miss the ball decrease the distance.
When you are consistent with these up to a reasonable distance, start over but contact the cue ball with various side spins. Start with half-tip out from center beginning with 1:00, then 2:00, then 3:00 and so on around the clock. Then repeat with the tip a full width out from center and repeat. Remember, to increase the distance only when you make the ball and decrease the distance when you fail to pocket the ball.
The same process applies to angled shots. The cue ball is at an angle to the object ball that goes into the pocket. Again, start with an easy angle at a comfortable distance from the object ball. On pocketing the ball, roll the cue ball a ball width to the side and repeat. On making the object ball, increase the angle a ball width. On missing the object ball, decrease the angle a ball width.
Like the straight in distance example, when you are comfortable with pocketing the object ball consistently, start over with the half-tip out from center, round-the-clock shots. Repeat this with one-tip out.
After you have all of the above, repeat the shots with one additional level of complexity. Place a full sheet of paper any place on the table. Play the shot so that you can get the cue ball to stop on the sheet of paper. See if you can figure out more than one way to do that. Then move the paper. It can be near or far as you wish.
Drills with multiple balls
The other set of drills involve placing multiple balls on the table in various formations and positions. The purpose of these types of drills is to force you to get the cue ball into prime positions to shoot the next ball in and then be able to get into position for the next ball in the pattern.
Some of these drills, when you first try them have hidden difficulties or reveal specific weaknesses in your game. Those shortcomings can make the drills impossible to complete. For example, the rectangle drill (below) can reveal many weaknesses in speed and spin control. Get out of line for just one ball and you rapidly find it impossible to recover. This drill alone can keep an obsessive person busy for several years. If you are masochistic in nature, there are dozens of drills like this available on the internet.
Drilling for reality
When you start off, concentrate on making small improvements of your current skills. Then set up shots to push your limits of competency and skill. Don't select drills that are too tough. You want to have some successes to encourage your willingness to practice. Concentrate on improving those skills that make you a better shooter within the group of people you currently play with and against.
Don't worry about shots beyond your current skill level. Work on drills and shots at the edge of your abilities. The advantage of using drills is that any improvements are obvious. A week or month ago, a certain shot would be tough. Now it is easy – thanks to your consistent effort.
Select drills that require improvements in areas that you want to improve. When you work on those areas that interest you, it is easier to observe advancements in your abilities. Gradually you will improve your status and move up the hierarchy.