The water supply line for my ice maker was never connected. So I want to know how to tap into the PEX water supply line to allow connection to the ice maker.
First you need to cut the PEX tubing at the access spot. That can be done with a PEX tubing cutter.
Then you acquire the correct type fitting (your size requirement may differ from picture) to rejoin the cut PEX with "T" fitting and a branch connection for an added PEX tubing to go off toward the ice maker. It is also necessary to install an inline valve to permit turning off the ice maker connection as shown.
An alternative method is to use a more conventional nylon tubing supply line toward the ice maker and connect that to the PEX tubing with a integrated valve type "T" fitting like this:
At the ice maker end of the newly added PEX line (if you had decided to run the distance with PEX) you would need to select the appropriate type of adapter fitting(s) to allow connection of the small diameter ice maker line.
At the actual ice maker connection it will be necessary to use the small diameter plastic tubing to the ice maker fitting. You can purchase kits that include fittings and the tubing at big box type home centers.