You need to use WiiScrubber 1.21 on a Super Smash Bros. Brawl ISO to insert those. I won't really get into that, you should already know HOW to do that.

WiiScrubber 1.21 is the slowest, but it's the safest.

All characters over at Partition 2 > Fighter

Anyway, the instructions are to make the PACs files, which require to be replaced too.

1)Get lz77ex ( http://www.patriotmods.com/brawlhacks/utilities/lz77ex.rar )
2)Drag and drop the PCS file into lz77ex
3)Wait for it to finish up
4)Rename the resulting .OUT file to .PAC (<character>.pcs.out becomes <character>.pac)
5)Insert into the game with WiiScrubber 1.21
6)Burn
7)PLAY!

Some characters don't really require the .PAC, but they are used in some Solo mode games (if you care).

The ones that do need them to work perfectly in normal Brawls:
-Bowser
-Wario
-Zelda / Sheik
-Samus / Zero Suit Samus
-Pokemon Trainer

I think that's pretty much it.

PS: PACs aren't released just because their filesize is like 3 times the filesize of a PCS. And since people can easily do those themselves, it's all good. Right?

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For the Pokemon Trainer's Final Smash, replace the FitPokemon##Final_en files relating to whatever costume. That sounds confusing, so lemme explain it a bit more.

Let's say you are gonna put the Metroid Trainer in with his Final Smash, right? Well, let's say you inserted all the costumes into the 01 of each PKMN Trainer character (PT, Ivy, Squirtle, Charizard). Well, you replace the FitPokemon01Final_en files. So now you have a 100% working and awesomely awesome Metroid Trainer, WITH FINAL SMASH TOO! :D