In one sense, chlorine is chlorine. So you can use Walmart bags, but you're gonna need a lot of them. They add either stabilizer, which you don't want now, or calcium, which is OK until it gets too high.

But bleach or liquid chlorine is the best. You can get blue 5 gallon carboys of 12.5% at most pool stores--that's like 10 gallons of ultra bleach.

You basically will run Free Chlorine way, WAY up to at least 15ppm and keep it there, checking it 2 to 3x/day to keep it there, running your filter 24/7, and backwashing regularly for a couple of weeks. Then, when it's ALL clear of Bacqua-glop., you will clean your filter. If it's DE, you'll take it apart like for a winterizing cleaning. If it's cartridge, you'll replace the cartridge. If it's sand, you'll replace the sand.

The process of conversion is incredibly simple, but that doesn't make it easy.