You Assume the shoes are worn. That's not a common thing. If it's squealing, probably loaded with dust. Might consider see grooves that push dust outboard. By decreasing shoe area you also make it not lock up so easy.
Taper the ends, run grooves across at an angle, ditch more than a fourth of the area if you have rear brake lock up.

The rear shoes last almost forever from wear.....however they can get contaminated with dirt and rust.  If you wash your bike and put it away wet, or if you get caught in the rain and put it away wet, or if it sits all winter under a cover or in a damp environment...the steel liner in the hub will rust.  Then when you next apply the brakes...the rust is going to be embedded into the brakes shoes.  (When you wash the bike and it is on the sidestand....the brake hub is leaning with the the open end tilted up and the closed end tilted down....it will hold water.  It is important you go for a ride and dry the everything out before putting the bike away).

So.....you can take the rear wheel/brake assembly apart.  Use some sandpaper/emery cloth to clean the brake drum and shoes....sand on the shoes until the glaze is gone.  Then clean them both with Brake Cleaner, and put them back together.  If you are really adventurous you can (should) take the shoes off, take the brake arm off and remove the brake cam from the hub, and clean and grease it.  If you want to use your new brake shoes....go ahead and put them on!

Everyone else has more experience than I but I too had a squealing rear brake. Moisture is for sure an issue and I'll still get it when it's been in the rain. But I've had a significant reduction of squealing after discovering the rear bolt was a little loose. After tightening it I have no squealing in dry conditions. Worth a check and a super easy fix.