I have not looked at my instructions lately, but the dry gas is used to remove the water. Use it right after you rinse the tank with fresh water. You can use the old pet cock as a plug as long as it does not leak.
Follow the instructions regarding water hose available and on and wearing old clothes and you should be ok.
If you want to test the acid before you use it, put some in a plastic bucket with something rusty and see how long it takes to work.