I am always amused by the responses I get when I tell potential puppy buyers that the pup they are looking at is litterbox trained, LOL. Reactions range from astonishment, amazement to disbelief and most often the "Why didn't I think of that!" response :o).
I hope you realize and appreciate the fact that a pup that already knows where to go to go potty, is halfways there with housebreaking!!!
Litterbox training works beautifully with young puppies, older puppies/teenagers and even smaller adults - you just need to adjust the size of the litterbox to the size of the dog. It works for me because I can leave puppies out to play and run about, and not have to worry about multiple poop piles and puddles to clean up every time I take my eye of them.
This photo above shows a 4 and almost 6 month old PekePoo using the litterbox, these are by no way small babies and they still continue to go there faithfully. Instead of a sweaterbox - for community puppy potty areas - I like to use "under the bed" storage boxes, as they are larger and can accommodate several pups going at the same time. If you know anything about multiple puppies - if one has to go, more likely than not - another one will need to go to!
Place the other litterbox next to the door you use to go take the puppy
outside [wether you go outside to teach "outside pottying" or because
that is simply the door you most frequently use to go outside with the
puppy. If you have a larger than normal house, or a two story home - be
sure to provide a 3rd box somewhere convenient for the little one...
As your puppy grows, gets older and more reliable with pottying and also becomes comfortable with the layout of your home, and gets settled in - it will tend to go to the litterbox on its own. But for starters - you will need to know when puppies are more likely needing to go, and be sure to take it there frequently and encorage it to go. Whenever you put it in the litterbox - praise, praise, praise when it goes, even if it is just a tiny little tinkle!!!
Once your pup is in the 12-18 week range, it can hold it better and longer, and they tend to anticipate when they have to go potty. And so will you - until then, you need to be vigilant and pay attention when they have the run of the house. These two boys here are doing very well with using the litterboxes, but I also make sure that they have ready access to them. Would I trust them from one end of the house all the way to the litterbox at the other end of the house with several doors in between that may or may not be open? No, of course not. The secret is either no doors or obstacles, or having several litterboxes in strategic locations.
I find poodle and poodle hybrid puppies to very easy to housebreak using the litterbox system. It lends itself naturally to either as a fulltime potty means, or as a transitional way of going outside to go potty. Not all dog breeds catch on to potty training equally well, but our kids here that have been started this way when they were still on mom or during weaning, are usually pretty solid - in consideration of their ages, and well on their way of being housebroken...