How to fill a grease gun, start by ensuring it's clean and free from old grease. First, if using a cartridge, remove the old one by unscrewing the retaining cap or releasing the lever. Insert a new grease cartridge, aligning it correctly, and secure the cap or lever back in place. For bulk grease, remove the barrel, manually fill it with grease, and then reassemble the gun. Next, prime the grease gun by operating the handle or trigger to remove air pockets. This process ensures smooth operation and effective lubrication. This is how to fill a grease gun properly for optimal performance.