Wrap Teflon tape 2-3x around the threads of the T-adapter in a clockwise direction. It helps with sealing the connection to avoid leaks!
Connect the enclosed steel hose to the SMALL end of the T-adapter. Use your hands (not a wrench) to screw it clockwise until it's as tight as you can get it. Avoid over-tightening.
Place the rubber washer into the top of the T-adapter with the cone-end facing up, as pictured below.
Use your finger to push it in firmly.
To avoid leaks, it's VERY important to screw the hose on flat and not at an angle. If you've aligned the threads evenly it should screw on quite easily.