This video describes the steps I use to litter box train my puppies using Pine pellets.
Breeders can begin litter box training as soon as they introduce food and take care to keep the pen to the appropriate size according to age. 1 foot per week of age.

My puppies are miniature sized Schnoodle puppies. You can visit my website at SchnoodlePuppy.weebly.com

The Litter box is a DuMor Plastic Rabbit Tray
https://www.tractorsupply.com/tsc/pro...

The Litter used is Pine Pellet Stall Bedding
https://www.tractorsupply.com/tsc/pro...

The PupEZ Whelping Box. The one I use is 3 X 4ft.
https://pupez.com/collections/whelpin...