When the leather starts to sag and give less support than it used to then it’s time to tighten it back up by lacing the sidewalls.
1. Measure the holes evenly (or somewhat close to) and drill both sides about a quarter of an inch from the edge. The size of the holes really depends on what kind of laces you intend on using. Six or eight should do- don’t drill an odd number of holes unless you want to tie off on the outside of the saddle!
2. I’ve seen people do some intricate lacing but whatever gets the job done. Certain areas can be tightened different to your preference.
3. Knot the ends or do something fancy to your liking.
4. Keep riding for another five years and then think about getting a new one.