Create a level spot to stand your second tank on. The location of your existing equipment will help you determine where to do this. You may have a well house with a concrete floor and just have to clear a space, or your well may be outdoors, in which case you may have to pour a concrete plinth or stand the tank on concrete blocks. Set up the new tank on its new base. The second tank should be as close as possible to the first tank.
Measure the distance from the inlet pipe of your existing pressure tank to the inlet pipe of the new pressure tank. Measure the diameter of your well pipes. Purchase a T-connector and an elbow connector. If you have copper or galvanized steel pipes you will have to measure what lengths of pipe you need and purchase threaded pipes of the correct length and diameter. If your well uses PVC or PEX plastic pipes, you will be able to cut them to length using a handsaw or power saw.
Shut off the power to the well pump at the main board. Drain all the water out of the existing tank -- there should be a faucet at the bottom for this purpose. Cut the inlet pipe to the old tank or undo the existing joints with a wrench.
Connect a new length of pipe to the new pressure tank by placing a T-connector on the pipe that goes into the first pressure tank. Lead a piece of pipe to another T-connector at the inlet pipe to the new pressure tank. Join them and fasten securely. For metal pipes wrap Teflon pipe tape on the threads before tightening with a wrench. If your pipes are PVC or PEX use pipe cement.
Flip the power switch back on. Your pump will fill both tanks and keep them at the preset pressure.