Maybe you have rescued or adopted an older dog, and you need to train him to use one spot in your yard to poop and only that one spot. Or maybe after years of living in a different home, you have moved into a new home and have decided you want all the dog poop in your yard to be in one place. Picking up poop in one small area and saving the rest of your yard for poop-free running and playing is a great goal to have. You might be retraining a dog you've been picking up after for years, or you may be training a rescue dog who is getting used to his new home anyway. Either way, If you have an older dog, you're going to have to train him to recognize the area where you would like him to poop every time he needs to go. Training older dogs is not difficult but will require some commitment from you both and lots of repetition and love.
Training your older dog to poop in one place will require you to first decide where you would like his special spot to be and introduce him to that spot and only that spot during potty time. This might also mean making your older dog hold it for a little while so when he has to go, he goes rather quickly instead of searching for the perfect spot across your yard. Consider the first several times taking your older dog to his spot on a leash to better control where he goes throughout your yard. As he gets used to the special spot, he can eventually go without the leash. You will need to do things like keep the area clean, especially if this is a smaller area than your dog may be used to, but consider keeping one pile around for a physical reminder and sense for your dog to remember where his special spot is.
To start training your older dog to poop in one spot, you will need a leash and an area reserved in your yard for your dog's poop. And lots of treats. Be prepared the first several times to walk with your dog to his special place, reminding him of the new rules and where to go. Every time you go out with your older dog to teach him or remind him of his special pooping spot, take some extra treats with you so you can reward him for remembering on his own or just for pooping in the right place once he's successful.