Heyyo,

     Aright, after you install BW's Mission Builder v4.7, it'll ask for ya GTA3 directory, obviously, point it to ya GTA3 directory. THIS IS IMPORTANT, mainly cause it requires some files such as the ingame text to determine what text it uses, or if you want to "compile and run" straight from the mission builder. (You'll use "compile and run" a lot to test your modding) After that, I suggest making a folder somewhere and call it GTA3 modding or something like that. Then, go into your gta3/data directory, and make a backup copy of the main.scm file, you never know when you'll need it again.

    OH! and before we begin, BW's Mission Builder uses pearl for coding, so it's easy. 

   NOTE:  This tutorial works also for VC, just use BW's VC Mission Builder, and the vc directories, etc..

     Now, open the main.scm in the Mission Builder, and it'll start to decompile it. Once it's done, you'll want to save what it decompiled to a file, preferably that GTA3 modding folder I told you to make before. Don't put it all in the GTA3/Data directory, or it'll get messy, trust me, My modding folder's still a bit of a mess cause it's all scrambled... lol.

     NOW, you're done savin' and decompilin', time to mod! :) Modding is not easy, but then again, it's not exactly C++, so be thankful for that, cause hell, even I don't know C++. Lol, maybe tha's why my modding's so unstable. 

     Anywho, to learn, I highly suggest that you compare the missions that are here in the decompiled main.scm file, to how you play them in the game. If you do that, you'll learn pretty fast, it's how I learnt to mod.

     Also, you can see how mods other people have made, such as mine, You learn pretty fast this way too. This is how I learnt to mod, and I started by modding BW's multi mod v1.0. This explains why my GTA3 mods are called Multi Mod v2.0 and v3.0. I learnt mainly from the Vigilante Aids, maybe tha's why I'm so fancinated with them? 

     So just keep at it, there's some readme's that BW included with the Mission Builders, go through those.

     OH! check the other files he sent ya, they all have detailed stuff that can prove to be useful, such as the opcodes.txt  This file shows you what opcodes the game engine can use. You'll probably have this and the decompiled main.scm file open at the same time, so you can quickly shift between the 2 files.

     Most of the opcodes have been discovered, so you can just fish through them all to see which one complements your idea.

     Also, be sure to visit www.gtaforums.com in the modification disscussion part incase other people have discovered more unknown opcodes. Incase what you need may just have been identified.