The schedule
Puppies need to go out on a predictable rhythm. Take your puppy out first thing in the morning, after every meal, after every nap, after every play session, and right before bed. Young puppies (8 to 12 weeks) also need a trip every one to two hours during the day and at least once overnight.
Go to the same boring spot each time. Stand still, stay dull, and give them up to five minutes. The moment they finish, throw a party: happy voice and two or three treats right there on the spot, not back inside. The treat must happen within seconds, outside, or the puppy connects the reward to coming back indoors instead of to going potty.
Supervise or confine
Accidents happen when a puppy is unsupervised, every time. Between outdoor trips, your puppy is either in your direct sight or in a crate, pen, or tethered near you. This is not forever. It is for the few weeks it takes to build the habit. Every accident your puppy practices indoors makes the outdoor habit weaker, so prevention is the whole game.
Learn the pre-potty signals: sniffing the ground in circles, squatting posture, sudden disengagement from play, heading toward a quiet corner. When you see them, move calmly and quickly to the door.
The accident protocol
When you find an accident after the fact, say nothing and clean it up. Scolding after the fact teaches the puppy nothing except to hide it better next time. If you catch them mid-squat, interrupt gently with a calm "oops," carry them outside, and reward if they finish there.
Clean with an enzymatic cleaner, not regular soap. Regular cleaners leave behind proteins the puppy can still smell, which says "bathroom here" to their nose even when you cannot smell a thing.
How long does it take
Most puppies are reliably house trained between 4 and 6 months old with consistent scheduling. Smaller breeds and very young puppies take longer because their bladders are tiny. Regression during exciting changes (a move, a new schedule, a vet visit) is normal. Go back to the frequent schedule for a week and it resolves.
If your previously trained puppy suddenly starts having accidents, especially with increased thirst or urgency, check with your vet. Urinary tract infections are a common and fixable cause.