You can buy a household door knob handle such as this one at a Home Depot or any home improvement store. This certain door knob is a basic bathroom handle that lock from inside the bathroom with the push of a button. check to make sure you have all of the right pieces such as the picture above has.

Step 1: Examine the Door

Before installing the door knob check to make sure that the place of the door were the handle is going to be put into that is is the correct size and structurally sound. Alomst all door come with this pre made and one does not have to worry but its good to check.

Step 2: Insert Door Latch

Is the part of the door that will be place inside the middle hole of the door. This is what make it so the door can stay shut and be able to be opened. With the latch and door stopper on the outside slide the piece in until it hits the piece of metal where the screws are placed.

Step 3: Screws and Placement

Take the two smaller of the 4 screws and place them in the hole above and below the latch. Make sure to check that the metal plate on the outside of the door is snug with the side of the dorr and does not stick out.

Step 4: Screwing in the Latch

Here you will need a star nosed screw drive to properly install the latch. Use the screw driver to drive in the screws with a gentle clockwise turn until they can't turn anymore. The metal plate should be snug to the door side with a smooth finish.

Step 5: Choosing a Handle

In the picture there are two knob the one one the left is for locking and should be inserted on the inside of the bathroom or the side of the door that you wan to be able to lock. the other is the outside door handle and is used for only opening and unlocking if necessary through the pin hole.

Step 6: Put Outside Door Knob Through the Middle Latch

On the outside door knob you will see how there is a metal rod or point. This is for turning the latch and should be inserting through the middle hole of the latch that was previously installed. After you are done put the locking door nkob on the other side by lining up the two handle for a snug fit.

Step 7: Screwing in the Knob

On the locking knob there are two hole for the screws. Take the remaining two large screws and insert them into these hole. Some door knobs are different and require the nail to be screwed in before the knob but that is not the case here. Take the star nosed screw driver and again turn the screws clockwise.