You will need to take them out on leash and stay with them until they go. Use a potty command as they are eliminating so they associate the act with the command. Give this command even as they are eliminating to further have them associate the command with the act then praise and reward with a treat that they will only get at this time.Soon they will associate the act, the command and the special treat with voiding out in the yard. To make it even easier for them mark an area in the yard with temporary fencing or surveyor flags so they have a visual of the are you want them to go in.
You will have to clean the deck with a cleaner that is specific to dog urine and feces so the odor is not there to attract them back.
Once they get the potty command down you can direct them from the door to the potty spot. Keep that command short ,two to three words, like "pen , potty ".
There's a product called natures miricle that I used on my deck.