I learned this when I first started playing and I still use it regularly. It's possibly the best way to develop reactions, wrist strength, and upper body stamina, and you can do it alone. You don't even need to be indoors if weather is not a problem.
First of all, you should have a smooth-surface wall (not brick or block) with nothing hanging on it or protruding from it (sometimes gym walls have plaques or flags), and the harder the better (concrete is the best--I often use a racketball wall). Second, the wall should be at least 12 feet high and 8 feet wide so you are not so confined and can work on different shots.
I use a nylon shuttle because of it's durability and it's bounce. I stand about 8 feet from the wall. (For certain drills, you can stand closer.)
Some of the things I do:
1. Underhand forehands--hit drives and clears using only wrist action. The drives would simulate a driving smash return to the forehand. If you hit drives, the shuttle will come back fast. When you learn to control it, the shuttle will rebound in rapid sucession; keep it going until you miss. If you're not used to it, you'll feel the tightness in your forearm soon.
2. Underhand backhand--same as #1, using the backhand.
3. Clears, forehand and backhand.
4. Smashes, forehand, backhand, round-the-head--set up the smash with a clear, underhand or overhead. Don't smash at much of a downward angle. If it helps, you can mark a line on the wall for the net tape.
Once you get control and stamina, you can create a rally:
1. Drives--switch back and forth from forehand to backhand.
2. Multiple smashes--set up for the smash, smash, smash the rebound, smash again if you can, and so on. This will train you to make quick wrist smashes and recover immediately, as you might need to in doubles.
3. My personal favorite: set up the smash with an underhand clear, smash, block the rebound, underhand clear the return, smash, and so on--clear, smash, block, clear, smash, block...Remember, although the legs do most of the work on the court, the upper body needs endurance training too. The rapid succession of shots here will work that. When you get to about 20 cycles without missing, you're doing good.
Extend the length of your workout gradually. Eventually, a good wall workout should last at least 30 minutes, even an hour. Keep going. When the shuttle hits the floor, scoop it up and keep going.
You have no opponent to watch, so this will help you focus on the shuttle. Keep your feet moving all the time, and practice your steps, weight shifting, and preparation prior to the hit. You need to practice proper technique because this will develop habits you will automatically use in a game under pressure.