Select the currency you'll be exchanging from on the left, so in this case ARPA, and the one you'll be exchanging for on the right, in this case QUAI.
Input the amount of ARPA you want to sell for QUAI, paste the receiving address for your QUAI after the trade is done and click "TRADE".
Finally send the amount of ARPA to the address that shows up on the screen by either copy-pasting the address shown on the page into your wallet or scanning the QR code on the right.
Sit back and wait
Make sure to note down the Trade ID shown on the page in case of issues with the trade.
Transactions can take a while to complete (often as long as an hour) so be patient.
If you encounter any issues contact Intercambio.app customer support with your Trade ID.