Hold back on the directional control to block either a high attack or an overhead attack. The only difference is you can actually just crouch and you will be able to totally evade a high attack, making your opponent whiff. Jumping attacks are generally classified as overheads so you need to block them high as well. Low attacks needs to be blocked crouching by holding down-back. Cross-up attacks needs to be blocked high and the opposite way around. Meaning you have to block standing and hold forward to effectively block a cross-up when you anticipate one being thrown at you.