Insert the cartridge of the new shower knob into the stem seat, and push it straight in. Re-install the nut, retaining clip, stem jacket and collar onto the shower knob cartridge.
Remove the stem cartridge of the shower knob by sliding the shower stem socket over the cartridge as far as it will go. Use your screwdriver like a lever by placing it diagonally under the socket and turning it to move the shower stem socket and remove the stem cartridge. Alternately, if your replacement knob came with a hub or collar that fits over the top of the old cartridge with a square top that you can grasp with a pair of pliers, insert the hub over the cartridge and use the channel lock pliers to unscrew the cartridge and remove it.
Pull the shower knob valve stem jacket or collar and plastic insert (if any) straight off the valve stem. You may need to grasp the valve stem jacket with needle-nose pliers to pull it off.
Turn the shut-off valve back on. Turn on the shower and look for leaks, both on the shower connection and at the shower knob. If you detect any leaks, tighten the shower connector by turning it a quarter turn with the pliers. Do the same with the nut on the shower knob.
Classic Shower Door Knob Pair