That's simple!
You just have to tap the "Unlock Tier" button on the bottom left corner of the Battle pass screen.
There you'll be able to control how many Tiers and how much it will cost in gems to skip them. :)
You can also see what are the rewards you're going to get for it. With the Premium Pass, you can unlock even more rewards, amongst other cool perks.
We hope you have lots of fun! See you at the Zoo!