We just put one in about a month ago, it is through the wall next to the slider and has 2 flaps. We started with pinning both flaps up for about a week, then letting it down on them as they came through. then after another week or so we put the outer flap down.
We had no problems with our dogs learning to use it. Even the smallest goes through and we never had to give any treats. We just gently put them through the first few times. Some of the dogs got it first time.
Unfortunately we still have accidents in the house when it is raining
I even have a retractable awning so they go on the deck if they want..