Tilt the weed trimmer so the opening to the propane holding tank is facing upward. Unscrew the cap to the propane tank.
Place a funnel into the opening of the propane holding tank.
Pour propane into the weed trimmer's holding tank until it is three-quarters full. Carefully remove the funnel to avoid dripping any excess propane along the sides of the tank.
Screw the propane tank cap closed, making sure it is tight and secure.
Read the owner's manual for instructions on how to start the propane weed trimmer; different types and models have different methods of starting. Most propane weed trimmers require you to press a primer pump five times, then pull on the starter string six to 10 times.
Put on your gloves and safety goggles.
Hold the weed trimmer at a 35-degree angle and move it slowly back and forth across the grass to create an even trim.