Start by undoing the chinstrap and removing the buckle. There is no need to remove the chinstrap from the bridle. 

You will see one end of the strap has less holes (generally 2) and this part is under the top layer of the strap so is the best end to alter as you won't see it once the strap is back together. 

You will need: 

1. A sharp pair of scissors
2. Either a soldering iron or a sharp pointed object like a sewing awl or a cooking skewer or small screwdriver.
3. A gas lighter to heat it with. 

Cut the 2 existing holes off the end of the strap with sharp scissors. 

After marking 2 new holes the same width apart as the ones you removed, heat the soldering iron (or tool) and pierce the webbing to make the new holes. 


Have a piece of wood or something not flamable underneath to protect the surface you're working on. 

Use the gas lighter to melt the end of the webbing - you can shape it with wet fingers while its still hot. 

Re-assemble the chinstrap so the buckle look like this with the roller end pulled tight,  and slide the plastic keepers in place.