I just downloaded the handling editor, and it seems more complicated than you'd think. It would be faster if you just opened the handling.cfg with Notepad. So the code he (Killa Killer) gives you is:
1600.0 4166.4 2.0 0.0 0.0 -0.2 70 0.57 0.98 0.54 5 340.0 70.0 2.0 R P 8.17 0.52 1 52.0 1.2 0.15 0.0 0.30 -0.10 0.45 0.25 0.3 0.52 19000 40000000 4 0 0 0
Then you open your handling .cfg, which should be in C:\Program Files\Rockstar Games\GTA San Andreas\data\handling.cfg. Now scroll down until you see the name of the car that you want to modify. In this example, I'm using the handling line for the Banshee, but it can be whatever vehicle you wish.
BANSHEE 1400.0 3000.0 2.0 0.0 0.0 -0.20 70 0.75 0.89 0.50 5 200.0 33.0 10.0 R P 8.0 0.52 0 34.0 1.6 0.10 5.0 0.30 -0.15 0.5 0.3 0.15 0.49 45000 2004 200000 1 1 1
Then you copy and paste the code that Killa Killer gave you. It should then look a little something like this:
BANSHEE 1600.0 4166.4 2.0 0.0 0.0 -0.2 70 0.57 0.98 0.54 5 340.0 70.0 2.0 R P 8.17 0.52 1 52.0 1.2 0.15 0.0 0.30 -0.10 0.45 0.25 0.3 0.52 19000 40000000 4 0 0 0
Do not overwrite the name of the car as it probably will cause your game to crash. Remember to always make a backup before you modify anything! Now once that is all done, start your game and check to see if it worked. If you need help installing other types of mods, please read this, this and this.