To install the casters:
  1. Lock the brake on the caster, and then tighten the caster into the conical leg. By locking the brake, it makes the caster its own wrench, and it is very easy to screw in the caster. 
  2. Tighten the caster until the nut makes contact with the cart or leg
  3. The caster has a thread-lock powder so a bit of force needs to be applied to fully install it. Note that by locking the brake, a wrench is not required for the installation.

If possible, try to install the caster only once and leave it for good. The more times you remove and install the caster the less of the anti-vibration material remains to help lock it in.