Wrap a layer of pipe sealing tape over the threads of the galvanized steel water pipe. Wind the tape in a clockwise direction, overlapping each turn approximately 50 percent.
Install the threaded end of the dielectric union onto the galvanized pipe. Turn the union clockwise and tighten with a pipe wrench.
Slide the union nut over the end of the copper pipe with the open threaded end facing out.
Score 2 inches of the end of the copper pipe with steel wool. Clean any existing burrs from the inside of the pipe lip with a reamer.
Install the brass union end onto the copper pipe. Be sure that the pipe is fully seated inside the brass end.
Sweat-solder the brass end to the copper pipe using a propane torch, acid-free flux and lead-free solder. Allow the pipe to cool.
Install the plastic insulator over the brass end of the union. Slide the union nut forward and turn it clockwise over the threaded union end on the galvanized steel pipe. Tighten using the pipe wrench.