Here is how you do it, Bull.
Make a folder on your desktop or whatever, called "Bull's GTA STUFF"
In there make a folder called "Modifications", and do the following:
Now in there, make three folders, they are labeled "Mods" "Backup" and "Other".
In "Mods" you store mods, in "Backup" you store the backups, and "Other" is for unrelated stuff.
Now go to this link and get Alci's IMG EDITOR, which is the best one in my opinion so far.
IMPORTANT: Be careful, don't click on the other "DOWNLOAD" things, they are viruses.
Go to the bottom where it says File Downloads, and it says alci_imgeditor.rar and click the small red Download.
IMPORTANT: YOU NEED WINRAR ARCHIVER for this to work, click on the link below to get WINRar Archiver.
Now, highlight all the files in the .rar pack and RIGHT click them, look for Extract to Specified Folder.
The window should pop up with many files and stuff in there, look for the "Bull's GTA STUFF" and click it.
Should extract the contents to the folder, delete the Alci IMG editor rar file if you want, saves space.
IF you did all correctly, then open BULL'S GTA STUFF and there should be Alci's IMG Editor in there.
After that, look for any mods you want, they mostly all come in .Rar files, which is why WinRAR is so important.
If you've found a mod you like, download it, and the .rar pack should come up.
Now, highlight all the mods you want, and right click, Extract to Specified Folder, and go to Bull's GTA STUFF and go to "Mods".
"Mods" if you did it right, should be located inside the "Modifications" folder I told you to make inside Bull's GTA STUFF.
After you got all the mods you wanted which is for example just ak47.txd and ak47.dff then close WinRAR.
After that open Alci's IMG Editor, and on the top left it should say File, click that and click Open.
Go to GTA San Andreas game folder, and go to models, and find GTA3.img.
IMPORTANT: Many mods are made for other IMG's, like cutscene IMG and stuff. Skins and guns, and cars will ALWAYS be in GTA3.img.
After that, search for ak47.txd and ak47.dff. When you find each one, click it ONCE and click the arrow pointing UP.
Search for Bull's GTA STUFF > Modifications > Backup. This is so you have a backup of all the files you are modding.
After you backed up ak47.dff and .txd (This is an example, could be any other file) click the arrow pointing down.
After that, search for the "Mods" folder and get the mods you wanted, in this case ak47.dff and .txd.
Highlight 'em both and click "Open" on the bottom, after that, click the save button, it's a blue floppy disc, then click REBUILD.
Rebuild is the machine like blue thingy, it should be rebuilding, when done, close Alci's IMG editor and you are done.