There's lots of ways to do it. I take the cube out of the freezer and let it defrost in a small dish which I then spoon small amounts out of, into the tank then keep it in fridge. The bloke at my lfs just holds the cube in the tank and swirls it a bit while some falls off. Some people defrost it and strain it through nets. Some people just chuck the whole frozen cube in
I wouldn't use hot water to defrost them as it may cook 'em a little. If you want to speed it up a little, dribble some tank water in.