Violin, Viola and Cello pegs eventually wear out or break. At that point you need a new set fitted.

1. The first step is selecting the pegs you like.
2. Then the peg holes are cleaned up a little so that they are round and if the peg holes are not exactly square they can be corrected slightly. If they are really off then they need to be bushed.
3. The shaft of the pegs need to be sized using a peg shaper so that the pegs are round. I usually leave about 5mm extra length so the final adjustment is hand fitted.
4. Once all the pegs are fitted and the peg collars are the same distance from the peg box. For violin I usually like about 12-13mm from the collar to the peg box.
5. Then the excess part of the peg needs to be trimmed off. Violin pegs look better if the peg protrudes about 1-2mm from the peg box. The end of the peg is slightly rounded and finished.
6. Appropriate size peg holes are drilled in the new pegs, usually about 1/3 on the winding side. The edges of the string holes can be slightly rounded using a small round file.
7. The pegs are doped.
8. The strings are wound like a screw towards the peg's head. This method of winding the string on the pegs allows you to adjust the tension on the peg and helps keep the peg from slipping. If they are new strings the strings will be stretching a little and the feeling of the peg will get tighter. After they stretch you may need to adjust the winding again.
9. Cello players sometimes like to "box" the pegs so that when the string is up to pitch the head of the peg is in the optimal direction. Once the string has stretched out, you can unwind the string and adjust how much string you feed through the peg. By changing that amount you will be able to change the direction of the peg head when it's up to pitch. With a little practice it's fairly easy.

Note: Change strings one at a time. Remember to dope the pegs, take a pencil and rub it over the string groves on the nut and the bridge. When putting on new strings or tuning you need to always check the position of the bridge. The back of the bridge needs to be perpendicular to the top of the instrument. Keeping your bridge straight will make it last for years and years.

Also when tuning using the pegs, if the bridge is never straightened, it will tend to warp because there is uneven tension between the string in front of the bridge and the tension behind the bridge. Straightening the bridge allows the string tension to equalize and the force is straight down. Even when tuning you need to check the position of the bridge because if you are not paying attention the bridge could start leaning forward and will eventually fall forward. Bridges that snap forward can easily break or even worse yet they could crack the top of the instrument.