Before starting any repairs, safety first! Turn off the water supply to the shower. Locate the shut-off valves—usually under the sink or in the basement. If you don't have shut-off valves, turn off your home's main water supply. Remember to always double-check that the water is completely off before proceeding.

Carefully remove the handle(s). Two-handle faucets usually have a screw beneath each handle cap. Use a screwdriver (or Allen wrench) to remove these screws and then gently pull off the handles. If they're stuck, a handle puller might be necessary. Take photos before disassembling to help with reassembly.

Remove the old cartridge. This often involves removing a retaining clip or nut. Use pliers or an adjustable wrench, carefully removing any remaining pieces and cleaning any debris from the valve body.

Install the new cartridge, ensuring it's the correct type for your faucet. Insert the new cartridge and secure it with the retaining clip or nut. Take care not to overtighten.

Reassemble the handle(s) and carefully turn the water back on. Thoroughly check all connections for leaks. If needed, tighten fittings and retest. Slowly restore the water supply.