I have glass bedded a lot of rifles, and it hardly ever makes a noticeable improvement, but I keep doing it, because it does not hurt, if done right. But if done wrong and tightening the action screws bends the barreled action, then it makes things noticeably worse. In that case, chisel out the bedding and start over.
It is much more likely to help a 338 Win Mag than a .223. The idea is to get the stock recoiling as a single mass with the barreled action. .223s will shoot fine without a stock.
I use Devcon steel putty, because I don't like to fight time or gravity.
I mask with masking tape.
I use Redding Imperial sizing wax as a release agent, but many household products would work.
I shim with layers of masking tape or paper.
I make pillars from 3/8" o.d. 1018 steel tubing, because the brass tubing has dissimilar metals problems and the stainless tubing is too hard to machine.