Netting rings come in a heavy or light variety. They are applied with a specially designed plier. For small or one-off jobs these are the most economical method.
For larger jobs or frequent use then it is worthwhile investing in a clip gun.
Two types are available:
The FP222 gun is a mostly plastic gun that is very popular with fencers. It works well and is cheap enough that if it gets lost amongst the long grass in a field, it does not matter.
The CL22 gun is a more expensive all metal gun, which is extremely reliable and will last for many years.
Refills of clips for both types of gun are available.