I have two... both are XL with a magnetic strip to aid in sealing out cold air in windy conditions.
The pups have to go though both to get out side.. door to back hall way has one, then door in hall way that leads to back yard.
No special training other then showing them they are there, perhaps a favored treat on the "other side" once of twice so they understand.
Only issue I have is when I put the solid "shutter" in place for wet weather or other reasons I want them to not have access to the back yard.. Once the pups get used to the door, they will go though them at high speed to get inside for dinner, outside to chase the squirrel.. ect .. if you just put the solid door in... the pup will end up smacking into it.. we avoid that by placing an obstacle in front of the door, chair, box, broom,, anything to make them slow down,, once the push against the solid shutter,, they know it's closed for the night/day,