How to install mods

First off, you're going to want a Program.

OPTIONS

• GTA Garage Modification Manager (GGMM)
  
• San Andreas Mod Installer (SAMI)
  
• Image Tool (IMG Tool)

Recommended

• GGMM - GTAGarage.com
  
• SAMI - Search it on google
  

Next, you're going to want a modification, for now let's get a vehicle mod.
Go to GTAGarage.com, then search for the desired vehicle.
Once you get to the mod's info page, click download.
It should have a .dff, and .txd, save those to in a new folder found on your desktop.
Then open up GGMM, or SAMI.
If you're using SAMI, just browse for the .txd, and .dff
If you're using GGMM, just click, and highlight the regular vehicle, then click installer, then browse for the .txd, and .dff

Then goingame, and you should see it!