The purpose of this guide is to take you through hacking Super Mario World. I will assume no knowledge about hex-editing (or that you even know what that is). However, I will assume reasonable familiarity with Super Mario World.
By the end of this tutorial, you should be able to rebuild Super Mario World in every detail. That is, you should be able to reconstruct SMW's overworld from scratch, and rebuild every level in SMW in every detail (auto-scrolling, smashing ceilings, everything). So not only can you recreate SMW, you will be able to create a game with similar mechanics as SMW, but with entirely different level layouts and so forth.
What this guide will not teach you is how to do things that are beyond the basic Super Mario World. The Lunar Magic editor for Super Mario World can allow you to change all kinds of settings about the game, add new graphics, etc. This is not the guide to help you do any of that. I may do another guide for that later.
You will need the following in order to use this tutorial:
Lunar Magic. This is the program that we will be using to edit Super Mario World.
The Super Mario World ROM. Lunar Magic can use the US version, Japanese version, or even the Super Mario All-Stars+Super Mario World version. Note that I cannot and will not tell you where to find one of these.
An SNES emulator to test your ROMs. I suggest ZSNES, as it is available on virtually every platform, runs almost perfectly, and is quite stable.
You should also be aware of one important fact: Super Mario World is not a game engine. It is not the Quake engine for 2D side scrollers. It was not built by Nintendo to be flexible. It was not made to allow for any game other than Super Mario World.
Some very smart people have significantly expanded on what SMW can do. Even so, you will come upon a lot of very strange limitations in this tutorial. In some cases, you will have to do some rather unusual or tedious things to cause certain effects. The only thing you can do is memorize them and accept them. These are the limitations that come with changing a game that was not intended to be changed.