The training room is a massively useful tool for practicing basic skills. A good rule with any training room exercise is to repeat it 10 times in a row. If you can't do it 10 times in a row, restart from the beginning.
Combos
One of the simplest things you can use training room to do is practice combos. Set the dummy to block after the first hit, so if you drop the combo, it will block, making it clear you've messed up the combo without checking the combo counter.
Reversals
Set the dummy to sweep continuously with a recording, when you are about to get up, do the motion and press the button exactly as the wakeup animation ends. If you see the Reversal message flash, you've done it correctly.
Meaty Attacks
In Street Fighter V, a good way to practice doing Meaty Attacks correctly is to set the dummy recording on Cammy, or another character with a dragon punch to perform the heavy version of that dragon punch as a reversal wakeup action. If you get the counterhit message, you did it correctly.
Hit Confirm
Choose a combo that can be hit confirmed and set the dummy to random guard. Only cancel into the special move you're confirming on hit, not on block.
Counter Hit Confirms
Check out the Hit Confirm article for examples of confirming on counterhit.
Set the dummy to block only the first hit, and to get counterhit, or perform a guard action, afterwards. When you counterhit them, follow the combo up.
Beating fireballs
Record the dummy throwing a fireball, then hold back when the fireball is done. Now switch to your character and try to find the timing where you can jump in and hit them without them blocking.
Whiff Punishing
Record the dummy jabbing twice, then doing the move you want to whiff punish. The jabs are your signal the dummy is about to do something, making it easier to time the whiff punish attempt.
Finding Frame Advantage
An easy way to figure out frame advantage for games where you don't have it available is hitting the dummy with an attack, pausing, setting the dummy to jump, then exiting pause while holding up. Both you and the dummy will jump, the difference in timing between you two indicates who has advantage.