First, you should locate the shut off valve for the lines intended on installing a T fitting in. Once it is off, you can take pressure off of the water line by turning on the water on a sink or appliance that the lines end. Then get a tubing cutter and cut the lines where needed. You can measure the inside diameter of the copper water lines and that will be the size copper T fitting you will need (For example, if it is a 3/4" inside diameter then most likely you will need a 3/4" x 3/4" x 3/4" T fitting) After cutting the lines, you will have to de-burr the pipes and make sure the opening is clean and square. I'm not sure how you would like to attach the fitting (either soft solder or press fit. Press fit is more expensive and you need a special tool) If soldering you will need to get solder as well as flux. and since the line is a potable water line, you cannot use lead solder. You can use a 95/5 solder (which is 95% tin and 5% antimony). You should dry fit the fitting to make sure everything lines up. You might need to cut an additional 1/2" off because of the fitting but don't forget to accommodate for the length of pipe going into the fitting itself. If you had to make an additional cut then remember to de-burr the pipe again. Once you have the pipe cut and everything is ready to solder, then clean the outside of the pipe on the male end (Add a 1/2" to the diameter size to find the right length of pipe you will need to clean, for example, if it is a 3/4" pipe then you should clean it 1 1/14" from the end) and the inside on of the fitting on the fitting. You can do this by using a piece of sand paper or a wire brush tool. Once the pipe and inside of the fitting is cleaned, DO NOT TOUCH! The oils in your skin will not let the solder stick to the copper. After it is cleaned, brush the flux onto the inside and outside of the male and female parts of the joint. Use a generous amount of flux but not too much. Now, place the fitting onto the pipe and twist it a little to make sure the flux spreads. Once fitting is inserted onto the pipe fully, You are ready to heat it up to solder it. you can use various kinds of torches to do so, but propane is probably the most common for this application. When heating the pipe, do NOT keep the flame in one spot on the pipe, move it around the pipe at the fitting. Test the temperature of the pipe by tapping the solder onto the seam to see if it melts and sticks to the joint. If it starts to stick, then back the torch off a little and hold the solder at the joint of the fitting and pipe while going the whole way around the pipe (the torch should be moving around the pipe as well with the solder and the torch should be at the fitting side of the joint because the solder will follow the heat of the torch and squeeze into the joint completely.) Once you went all the way around the joint with the solder, take the torch off and quickly wipe the joint with a clean cloth to take off any excess solder. Once it cools enough, inspect the joint for any holes in the solder that will leak. If there is a hole then apple flux and solder again. You might wanna practice a little, just to get comfortable with it if you never done it before. Practice always makes perfect.