Pull the stopper off of the vent, pull off the pressure tube, and fill through the uniflow. Sometimes it's easier to pull the pressure line at the muffler, sometimes at the tank.
Don't forget to put the vent plug back on when you've fueled it up.
You don't need a check valve -- on the outside of the tank, aim the uniflow line up so that the loop of tubing to the muffler can't form a siphon. Or never leave the thing fueled and waiting unattended.