There are guides for this kind of thing
To make a stage texture, you need to open the stage in Brawlbox (If you don't know what this is, I
highly recommend that you check out those guides). Once you have done this, navigate to texturedata[0], export anything you want to edit, and alter it to your heart's content in the image editor of your choice (my editor of choice is GIMP).
As for models (I'm assuming you mean vertexing), the methods that are out there aren't all that easy, or functional. You could try Stickman's tutorial if you really want to learn, otherwise I'd just wait 'til DasDonkeyVertexBox 2.0 comes out.
If you want to make 2d stages (take any random picture and turn it into a stage), check out WackaAlpaca on YouTube. His tutorials are the best thing out there for n00bs.