By moving clumps of grass in pokemon emerald.
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firstly use cut then by using secret power (its a TM)
Use the move cut to remove them, but not all things can be removed such as trees, boulders and fences. And not houses of course
Stand at the front of the clump of grass and press A but you must have a Pokemon with you that knows the move secret power and it should ask you if you want to create a secret base there. Hope it helps :)
You can't its not a move in emerald.
A HM is a move is a move that can be tought to as many Pokemon as you like but the Pokemon has to be able.