Visit the collection page to get the price, then simply send that amount to the address listed on the page, and a randomly selected NFT from the collection will be sent back automatically. The random number is generated using the VDF of the transaction block + the coin id of the received coin. A small fee is included to speed up the NFT transaction being accepted into a block.
If the collection is sold out and you send XCH to it, or if you send the incorrect amount (too low) the same amount will be returned to you (without a fee, so may take time to come back). If you send too much XCH, an NFT will still be sent, and the difference between the amount sent and price will be returned.
Any trouble, comments, or suggestions, reach out to scrutinous on keybase (will have the gacha.space proof in the profile) or scrutinous#2897 on discord where you can find me in many chia related servers.