It's actually quite simple.
To buy crypto, select the currency you'd like to purchase on the screen, and then press "BUY". The terminal will ask you the quantity of crypto (in USD) you'd like to purchase. From there, scan your wallet QR code (public key) using the ATM's scanner, and insert bills until you've reached the exact amount of crypto you want to purchase. Then hit the "Buy Bitcoin" or "Buy Altcoins" button on the bottom right of the screen and you're all done! The transaction will immediately broadcast to its respective network and you'll see the crypto en route to your wallet.
To sell crypto, press the BTC or LTC button on the screen, and then press "SELL". The terminal will ask you how much cash you'd like to withdraw. From there, select how much cash you want using the $20 and $100 increment buttons ($50s may be available at select terminals). After that, the machine will print a redeemable voucher with a public key QR code on it, and will also tell you how much crypto you'll need to send to that address. Once you broadcast the transaction, you'll need to wait for 1 network confirmation before you can actually withdraw your cash. When it confirms, go back to the terminal and select the "Redeem Ticket" button, and scan your voucher. After a few seconds, your cash will be dispensed!