Clean inside the tee on both ends that will be soldered. Clean the pipe ends as well. Flux both tee and pipes and assemble the joint. Solder both joints in the same operation. You heat the tee, not the pipes and when one side is hot enough to cause the solder to flow, the other side will also be hot enough so a quick wipe of the solder around both side is all it will take. Most homeowners use a propane torch although Mapp gas is hotter and faster, it also costs more money. May not be worth the added cost for the average homeowner.