Crate training style litter box training is probably one of the most effective way to litter box train a dog.
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Traditional crate training involves putting a dog in a kennel when you cannot watch it. The concept is fairly simple to which no animal will soil the area to which it eats and sleeps.
The crate style method for litter box training is similar, except eventually, the dog will have more freedom, even while you're away from the house.
Before you start crate style training, you should already have the dog comfortable with the litter box. The dog should also know that he is expected to use "his area" for the bathroom.
When litter box training using this methos, you aren't using an actual crate but a small "crate area." You can create an area that has just enough room for the dog, his bed, food/water bowl, and litter box. The basic concepts of the size of the crate still applies, to which the dog must have just enough room to sit, lay down, stand, and turn around, except there must also be enough room for the litter box.
The cage should be sturdy, as should the crated area, if you chose not to use a crate. In the later case, choose a material that the dog cannot jump over or hurt himself trying to escape from. A dog gate works well.
If using a puppy pen or cage, be sure to fill any open space with some other safe material that will prevent the dog from using open space in the pen or cage as a bathroom area.
Once you have constructed your "crate area", place the dog in this "crate" at typical bathroom times. The dog should stay in this crate area until he uses the bathroom.
Try to be nearby so you can catch the dog in the act and praise him right after he uses the box.
The crate area should also be the where the dog stays when you leave the house or cannot watch him.
When you are with the dog in the house, the crate area should be left open, and part of his "direct path" to his bathroom area.
If the dog uses the bed as his bathroom spot, wash the bed and sprinkle food and treats on the bedding. Because the dog will not use it as his bathroom area if there is food on it.
The crate training routine should continue until you see the dog use the litter box on his own.
You should try stay in the room with the dog at times when you know he needs to go.
By leaving the crate area, it will encourage him to use the litter box, repeating with the cue frequently, and praise him whenever he uses the box.
Soon, you'll be walk with the dog into the room, use the cue, and the dog will go to the litter box to do his business.