I use MTGPress.net for printing my proxies. When you want to print the backside of a card, you type in "1Back" to get the backside of a card. Then you just need to use your photo editing skills to line up the front face of the card with the back face. When you go to print, change the setting to "print on two sides"
Alternatively, the easier way. Print one side like you usually would, and then slip the proxy in a sleeve with a basic land. That way it also has the rigidness of a real card.