You can't, unless you mean secret base.
You have to have a Pokemon with Secret Power.
Use Secret Power in front of a tree or an indention in rock.
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its not an emerald its RAYQUAZA the main legendary Pokemon in Pokemon emerald.
You cannot get a Yanma in Pokemon Emerald, as only Hoenn Region Pokemon can be found in Pokemon Emerald.
There is no Autosave in Pokemon Emerald.
defog is not in emerald,
rotom isn't in Pokemon emerald