Instructions
Teach the "Cut" move, also known as HM01, to any Pokemon that can learn it. To see if a Pokemon can learn the move, pause the game and select "Cut" from your list of moves. Choose to teach it to a Pokemon. If any of your Pokemon can learn the move, "Able" is displayed by their names.
Look for any tall trees that seem to be blocking a path. Walk up to them. Face the tree you want to cut.
Press the "A" button. A prompt appears, saying that the tree may be cut. Press "A" again to select "Yes," and the Pokemon cuts the tree down for you.