1. Teflon tape (it is not recommended to use thread sealant)
2. 19mm wrench for th. And a 24mm for Mosmatic couplers, or 1" for MTM couplers. DO NOT use pliers, or adjustable wrenches. You will NOT be able to get the connection tight enough.
3. Workbench with vise is preferred, but not necessary.
1. Put 7 wraps of Teflon tape on the male 3/8" end of the adapter.
2. Use the 19mm wrench on the adapter, and brace on the ground or clamp in a bench vise.
3. Use the 24mm (or 1") wrench to install the coupler by pushing against the other wrench. You will need to really put your weight into it and get it TIGHT to prevent leaking.
4. Connect to pressure washer and gun, check for leaks.