Litter pan training begins once a puppy is weaned and apart from momma full time. The puppy moves to an exercise pen with a few litter mates and begins eating solid food full time and learns to use a litter pan. (There are links below for both the pans and pens.)
The puppies have a bed and food/water on one side of the pen with lots of chew toys to play with. On the other side of the pen is a litter pan filled with paper pellet litter pan material. It is known as Yesterday's News or PetCo's generic version is soPhresh.
It only take a few days of hovering over the pen to train the puppies to use the pan. They naturally want to "go" away from their bed and food. Once they use the pan a few times - they get it - and remember it for life!
The wonderful thing about puppies learning to use a pan as a puppy, is that they will always remember the pan. It you must be away, even when the bulldog is older, you can put a pan in the room and not return home to "accidents".
My first English Bulldog, Tug, proved he remembered one day after a storm had blown a trash can lid into the back yard. It looked enough like the pan he had learned to use years before so.....guess where he "went" that day?
You can find these exercise pens at Petco or PetsMart or other pet supply stores. The pens are 24" high with 8 panels that can be formed into any shape that fits your area. I often find these cheaper on-line than in the stores.