Place the soaker hose fitting, open side down, on a sheet of gasket material. Purchase gasket material in sheets from a hardware or home improvement store. Trace around the edge of the fitting using an ink pen.
Draw a circle inside the one you traced from the fitting. Make the inner circle the approximate size of the opening of the hose in the fitting.
Cut out the inner circle first using a utility knife. Take your time and ensure that you cut on the line you drew.
Repeat the cutting procedure for the outer circle. Place the washer into the fitting and make necessary adjustments with the utility knife to make it fit.