How should i potty train my Puppy?!?!?!?

Okay, so i got another boxer puppy. He is my 3rd and he has been a bad experience so far. My 1st dog toook like a week to potty train and it was pretty easy. The second dog just knwq how for some random reason when he saw our other dogs doing it. Now the puppy has no clue. He goes to the bathroom outside and we praise him. He goes to the bathroom inside and he is punished, but he just doesnt seem to get it. Is there any way that you KNOW willl work for our situation? Please help!!!