You'll use the knife to cut away where the water will flow into the spindle and the holes where the water will fall out and onto your head. The result is a shower head that doesn't look too sharp ...

Step 2 To remove all the old showerhead fittings, unscrew hose and showerhead, then use wrench to remove nut. Remove shower bracket and cover plate so wall spindles are exposed.

First and foremost, make sure your new single-handle faucet is even compatible with your shower valve. If it's not, you could be looking at a full valve replacement, too.