Cane it can be a process but it can be done. Its often best done in the spring/summer time since your going to be out there with them. Its a process but training any behavior takes time and making it a point to go through with it.
First off you have to designate a specific area for them to go. I've already got an area marked off with some short decorative fencing. Its more for people so they don't go walking through and step on a "landmine" but it does help give the dog(s) a "place" to go to so they know where to go. It doesn't have to be something special, so long as its something they can distinguish. Tall grass, a certain cluster of trees, a pile of gravel, a short fence etc.
Next you need to get some small treats that the dog(s) like. Keep them handy and the dog ONLY gets them when they do their business in that area. I use a ziploc sized sandwich bag in my pocket with the treats. When we go out the bag goes into the pocket when I'm done the bags put up but its always got treats in it to help make it easier for me to get out with them.
Then you have take them out on leash and wait with them while they go. Take them to the area you want them to go and stand put. Your literally an anchor. Its ok to pivot but don't wander around. You keep the dog inside the area you want them to go, and just wait. They literally can only roam around this area because the leash is only ___ long. I like a 6ft leash but that's just me. Its honestly better to use a regular leash than one of the retracting ones so your not tempted to let them have more length to go off and wander around. The more they can wander the more likely they are to not go where you want them to and the longer it can take. If you only have a 12ft circle you can sniff they'll quickly smell all the smells and then decide its time to go since Mom's not letting me wander around. You just have to be firm about not wandering with them. Stay in the area, they'll go quickly enough.
Soon as they relieve themselves, you have to praise them and tell them how great they are doing. Use a silly happy voice. Get animated like them going there was the best thing they could have done. Seriously. Give them a treat. And when they go in, they go back inside on leash. Then they can be off leash and hang out. Even if its just #1 lots of praise. If its #2 even better. Every time they go outside to go potty, clip the leash on, take them to their spot, you stand put keeping them in their area and PRAISE PRAISE PRAISE & treat
If they are going to go outside with you and hang out, then make them go there first to go. Leave them on leash. Let them drag the leash, it won't hurt anything. If they start to go, snatch up the leash, and run them over to the area you want them to go in and again praise praise praise & treat. Usually once they get moving they'll stop going since they often don't want to walk and go. Get them in the area and wait till they go. Even if its a little bit since you caught them going elsewhere its a big deal because they went where you want them to go. Again praise and treat.
It takes a little while. It can be work and with multiple dogs its that much more work. Soon you'll create the behavior where they go where you want them to. Your just creating a new behavior. No different than sit or anything else. The more you work with them on it the faster they'll get it. If your not consistent it will take even longer.
At our old house (before we bought this one) we taught them to go to the edge of the weeds to go. No stopping between here and there, you go out there to go. It was tall grass and they know run out there and they go just fine. Took a little bit but soon it was just an ingrained behavior and they know I go potty there. Its no different than housebreaking. They learn they can't go in the house, and they learn go over there when its time for me to go.
If you take them out and its been a while and they still won't go put them up. Be it in their crate or in the house. Try again a little bit. Same steps leash on, walk them to the target area, and stand with them. Bring them back in and leash off.
Once they start going there on their own you can take the leash off and encourage them to go there. Still praise them up. Then you start just dropping back and they go while you stay at the door. Praise when they go, and praise when they come back. Next thing you know they are out in the yard with you and they'll walk over there and go on their own. Our dogs do that now and its wonderful.
You'll be so glad you put the work into it. Its easy to do, just takes a little time and effort. Then you can put one of these
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=570 near by and your all set. Clean ups a breeze just be sure the hose is close enough to "flush" they system.
Hope this helps. If you have a question or a problem don't hesitate to ask