In order to Xyz Summon, you need two or more face-up monsters on the field with the same level. You’ll also need an Xyz Monster in your Extra Deck that fits that Level. (Example: If you’re using 2 Level 4 monsters as your Xyz Material, you need an Xyz Monster that lists “2 Level 4 monsters” as the materials.)
When you’re ready to Xyz Summon, just take the Xyz Material from the monsters and pile them up on the field. Then take the matching Xyz Monster you’re Summoning out of your Extra Deck, and Summon it. Then place the pile of Xyz Materials underneath it.
That’s right: The Xyz Materials go UNDERNEATH the Xyz Monster. Then you can use them to pay costs for Xyz Monster effects, or to activate special Spell and Trap Cards that only work for Xyz Monsters!
Whenever you use an Xyz Material to pay a cost, or activate an Xyz Monster’s ability, the card will tell you to “detach” 1 Xyz Material. That means you take one of the Xyz Materials you stacked up under your Xyz Monster, and send it to the Graveyard.