I see Noob sliding A LOT and abusing the glitch(on his wr) but how do they double jump without the power-up? also how do they get so much momentum?

Take a look into the guides. Its explained there. He can double jump by using gdj and the momentum is prob crouch jumps

oh thx I didn't even notice the guides lol, I'll look at those thx!

By sliding I assume you mean spamming crouch in order to get upwards of 54m/s when jumping. You can do that by spamming your control button by moving your hand off of you mouse onto your control button, and then it's pretty easy. If you mean sliding then jumping then crouching, it's not too hard. All you have to do is slide and crouch jump, as that your crouch jump begins with more speed.