Beginner Exercise 1 - Highs and lows
The reflex of blocking a jump attack high and then transitioning to a low block is an important habit to build, there are ways to extend air attacks to stay in the air longer so as you improve try to delay the crouch as long as possible, for now just do it right after blocking an air attack.
Jump into training mode against a character you know fairly well.
Hit the record button and jump and land a heavy attack before you hit the ground, as soon as you can use your fastest grounded low attack, usually 2L but for some characters it's 2M (If you aren't comfortable doing this yet, I'd recommend taking the time to practice, it's an excellent tool for offense).
Position yourself close enough that the air attack will connect (or move to the corner) and hit playback. Block the high attack and then crouch low to block the low attack.
Repeat this until it starts to feel natural and swap sides regularly.