First, make sure you have a compatible printer. If your printer is on that list, you'll need to tell Square to use it as a receipt printer.

Under Settings -> Printers on your app, create a printer station. Make sure you have enabled "Receipts, Bills, Reports, Open Cash Drawers". More info on creating printer stations.

Ring up your customer as you usually do. At the end of the transaction a receipt choice screen should pop up. If the printer is configured correctly one of the options will be to Print Receipt.

If this receipt screen does not pop up at all, more than likely your customer has entered their email address or phone number with another Square merchant at some time. The receipt will automatically go to their entered information. If the customer still wants a paper receipt, tap Transactions. The most recent transaction will be the one that shows first. Tap New Receipt, then you can print the receipt from there.

If you still have printer troubles, check out this help article.

Hope this helps!