I'm a little confused on the ability cascade
The first sliver costs 5 mana
Do I exile until I find a 4 mana, 3 mana, 2 mana, 1 mana
Or do i exile until i find any creature and then go down from that creatures cost?

you cascade revealing cards from the top of your deck until you reveal the frist nonland card that costs less than 5 mana and you can cast it if you want. If you dont want to cast it it goes to exile and the rest of the cards go to the bottom of your deck

I watched a video that kinda showed me, he casted the first sliver but when he came to a sliver he casted that and then casted it again.
Does the haste or flash sliver allow you to cast it multiple times?

If all slivers have cascade then you cast the second sliver revealed if it's lower cmc (converted mana cost) of the one you've just cast