Okay, I know how it says K R MA, but it really stood for King+Rook Mate. So, what you do is you put your king somewhere in the middle below the rank of the rook, and the other king is one rank above. It's hard to explain, but a diagram might help.

So, when the kings turn into opposition, give a check! Now NO ROOK SANDWICHES! So please don't do this:

Instead, give a far end check. Now, when your opponent's king gets close to the edge, immiediately go on the file on his side with your king. Yes, but what if you can't get there in one move? Keep moving there! But if the king gets close to your rook, don't protect it! Move it to the farther end of the board. And this is a perfect checkmate: