What is the basic procedure to install a brass compression tee into an existing 3/4" copper water service line?
It doesn't have to be a copper tee, but if it is what you bought so be it. Make sure that you have a male or female NPT on your outlet port. Cut a piece out of the copper main that is not wider than the width of your outlet port. slightly bend one side of the mainline and slide the end fitting and gasket on the one side all the way on, do not tighten. Slide the second end fitting and gasket on the opposite side. Then bend the pipe back in line with other side of the mainline and center the tee. Slide the gaskets and end pieces onto the tee and tighten down with a wrench. Screw a cap plug to the out let port of the tee and test for leakage. Tighten anything that leaks. When that past leak test. Unscrew cap or plug and install a threaded nipple if female or a threaded adapter if male. Then attach a ball valve to be able to shut off the entire irrigation system if there are repairs to be made later but most important you can turn back on your mainline to the house and continue working or your system at your convenience. I would do everything in PVC because it is easier and more convenient to work with. Make sure you pt concrete or something that can withstand thrust at all 90 degree bends. JMHO. Aloha