Go to https://puzzleswap.org/trade

Connect your wallet: the button is in the top right corner.

Trading screen: top asset is the asset you want to pay with, click the drop down to select the asset, fill in the amount you want to spent, the bottom asset has to be BURN-XTN and the number on the right is the approximate amount you will receive.

Click 'swap button' and your done. (depending on the wallet you use, you might have to confirm the transaction).