Shut off the water supply and drain the pipes. Use a tubing cutter or reciprocating saw to cut out a section of horizontal pipe long enough for a T-fitting.
Install a T-fitting as close to the valve as possible.
Install a short piece of pipe in the branch arm of the T-fitting. This short pipe will be used to attach a threaded fitting.
Install a threaded fitting. Use a fitting recommended by the manufacturer of your arrester.
Wrap the threads of the arrester in Teflon tape. Thread the arrester onto the fitting by hand. Tighten by holding the fitting with one adjustable wrench and turning the arrester with the other. Do not overtighten. Turn the water on and check for leaks.