I tried it a couple of weeks ago to get to Crescent Isle. It does not work that good. The only thing you can change is the number of the Map, the coordinates do nothing when edited. So you'd need to have a decrypted ROM to determine which map ID you need with the tool "RSWE". Save anywhere, edit the ID, load your save file, see if it worked, it's that "simple" The problem is that most of the times you do it like this you will just see a black screen when you load the save file so you need to edit it back to its original state. Also, I don't know if you will actually be able to encounter wild Pokémon like this, because when I finally was able to "warp" myself onto Crescent Isle the ring you need to interact with to encounter Cresselia was moved forwards a bit, so you simply couldn't access with.
There's an easier method to being able to fly to multiple Mirage Spots though and that is Homepass, just google it