The module files in Super Smash Bros. Brawl are located in the module folder. All fighters and stages have these files, which have the suffix .rel. For example, st_donkey.rel is the file for 75m.
Editing stage .rel filesEdit
You will need a hex editor.
Open the stage module file (of the format st_XXXX.rel) for the stage you want to replace something with. Let's say you want to replace Flat Zone 2 with a stage based on Final Destination. Each module file has a sequence of bytes somewhere in the file that goes like this:
38 A5 00 00 38 80 00 XX
The value of XX is the ID number of the stage, taken from this list:
STCUSTOM (Phantom Wings' code) 00 st_custom[1-9].rel Battlefield 01 st_battle.rel Final Destination 02 st_final.rel Delfino Plaza 03 st_dolpic.rel Luigi's Mansion 04 st_mansion.rel Mushroomy Kingdom 05 st_mariopast.rel Mario Circuit 06 st_kart.rel 75 m 07 st_donkey.rel Rumble Falls 08 st_jungle.rel Pirate Ship 09 st_pirates.rel Norfair 0B st_norfair.rel Frigate Orpheon 0C st_orpheon.rel Yoshi's Island (Brawl) 0D st_crayon.rel Halberd 0E st_halberd.rel Lylat Cruise 13 st_starfox.rel Pokémon Stadium 2 14 st_stadium.rel Spear Pillar 15 st_tengan.rel Port Town Aero Dive 16 st_fzero.rel Summit 17 st_ice.rel Flat Zone 2 18 st_gw.rel Castle Siege 19 st_emblem.rel WarioWare Inc. 1C st_madein.rel Distant Planet 1D st_earth.rel Skyworld 1E st_palutena.rel Mario Bros. 1F st_famicom.rel New Pork City 20 st_newpork.rel Smashville 21 st_village.rel Shadow Moses Island 22 st_metalgear.rel Green Hill Zone 23 st_greenhill.rel PictoChat 24 st_pictchat.rel Hanenbow 25 st_plankton.rel ConfigTest 26 st_config.rel Result 28 Temple 29 st_dxshrine.rel Yoshi's Island (Melee) 2A st_dxyorster.rel Jungle Japes 2B st_dxgarden.rel Onett 2C st_dxonett.rel Green Greens 2D st_dxgreens.rel Pokémon Stadium 2E st_dxpstadium.rel Rainbow Cruise 2F st_dxrcruise.rel Corneria 30 st_dxcorneria.rel Big Blue 31 st_dxbigblue.rel Brinstar 32 st_dxzebes.rel Bridge of Eldin 33 st_oldin.rel Homerun 34 st_homerun.rel Edit 35 st_stageedit.rel Heal 36 st_heal.rel OnlineTraining 37 st_otrain.rel TargetBreak 38 st_tbreak.rel
So in the example, you would want to open st_final.rel search for:
38 A5 00 00 38 80 00 02
You want to replace the XX value with that of the stage you want to replace. So change the string to:
38 A5 00 00 38 80 00 18
Save the file as st_gw.rel. Now you should be able to put a Final Destination-based stage over stages/melee/STGGW.PAC.
For custom stages using Phantom Wings' Custom SSS code, the XX value you should enter is 00.
Also, Smash World Forums user Bero has given us a tool to easily make .rels! http://www.mediafire.com/?rmr2j837li4867n
A .rel file to use an OnlineTraining-based stage as a custom stage with Phantom Wings' code can be found here: http://www.mediafire.com/?gkeikwjjqz1 In this file, XX is 00, but you can change it with the method above if you want it to replace a stage already in the game. This .rel file is verified to have no Sandbag. (as far as I know, this is something that PW changed in the .rel, but I might be wrong --User:Libertyernie)