If the air pressure is lower, using a compressor add air, and if it is higher, just release air at the schrader valve on the tank. Do this by letting air out of the tank or pumping it in with a tire inflater. One cannot set up a precharged tank according to this diagram. After that, all you do is look at the pipe diameters you currently have and look at the opening diameters of your new tank and get all the pieces to fit. In this example there is a Johnson Controls' F93B.
That is your cut-in pressure.
How to Replace a Water Well Pressure Tank
So it took some cutting soldering etc. So, I'm making a list for the hardware store to get some tools, teflon tape, maybe sediment filter replacement, pressure relief valve, and a new pressure gauge among other things. While the diaphragm was working, the system worked great. This way, the momentum of the travelling water will help it to continue travelling past this tee whose purpose is to allow air from the top of the tank to pass back to the well. It's best to remove the circuit breaker for the pump completely, as this prevents any unfortunate surges. You will need this. Before you begin to do anything else, you should ensure that your water and electrical are turned off at the well.