There's no real need for an uptodate texture tutorial because it is extremely straight forward.
Not sure if you've saw it yet, but I direct you to the noob guide (it sounds like you haven't....). It covers all of this in more than enough detail.
http://forums.kc-mm.com/index.php?topic=12169.0In particular:
Scroll down to where it says "A. Gecko OS". That tells you how to set up the gecko codes.
Character textures go in (X):\private\wii\app\RSBE\pf\Fighter\*charactername*\
Download a texture and place it in the correct folder for the character in question. To put a downloaded texture over Marth's default costume for example-
*download a random Marth texture
*rename it fitmarth00
*make sure that it is in .pcs format
*if not, open it up in Brawl box, go to file/saveas and save it as a .pcs.
*go to the fighter folder and place the fitmarth00.pcs file in the Marth folder.
*load up the game through gecko and thats it
This is further explained in the guide.
Stages go in (X):\private\wii\app\RSB(X)\pf\stage\melee\
You said that you put them in the "stage" folder. That is right, but they need to be in another folder inside stage, called Melee. That might be your problem. Stages go in the "melee" folder, which is inside of the "stage" folder. Certain stages require .rel files as well so watch for that.