Hold the Hilti gun so it is pointed away from you and slightly down at the ground. This way, should it accidently fire, the nail will drive into the ground and not fly through the air, where it could cause injury to someone.
Press the release tab or button (depending on your model) at the base of the handle grip to release the charge cartridge. Pull the cartridge out of the grip. Place a charge strip into the cartridge. The charge strip will snap into the grip along the advance rod that is sticking up from the base of the cartridge. Your strip should extend straight up out of the base of the cartridge and along the advance rod (the advance rod is what will hook and pull the charges into the firing mechanism of the gun). Push the cartridge back into the grip, make sure that the charge strip goes into the grip with the strip toward the trigger side.
Load your nails. On single-shot Hilti models, you will load one nail at a time by pushing the head of the nail into the tip of the gun until it "clicks" into place. Make sure you keep the tip of the gun pointed away and down while you do this and that your hand is not over the nail as you insert it into the tip. This way, if it fires accidentally, it will not go through your hand. For models that use a nail strip, pull the load release lever on the top of the barrel of the gun near the tip back to open the nail slot. Insert the first nail of your strip into the slot with the head of the nail facing towards the rear of the gun (the grip end) and push the lever forward until it locks. Your Hilti is ready to be used.