I am assuming you already have BURN-XTN token in your wallet, otherwise you need to buy it first, see How to buy BURN-XTN? page.
Connect your wallet: the button is in the top right corner.
Input the amount of BURN-XTN tokens you want to add as liquidity.
Click the 'deposit button' and your done. (depending on the wallet you use, you might have to confirm the transaction)