Lay the office chair out on its side so the legs are easily accessible. Remove any worn casters before you begin installing the new ones.
Insert a new caster into the first leg on the office chair. Push the caster in as far as it will go. You should feel the caster snap into place when it is fully seated.
Add the remaining casters to the legs of the chair. Test each caster to make sure it is tight and firmly seated.
Place the chair upright and sit in it as you normally would. Roll the chair around for a few minutes, then place the chair back on its side and check the tightness of the casters again. Tighten the casters as needed.