You can install this mod in 2 ways.

1 the easy way
open data in gta 3 main directory
drag the files (carcols handling and default) from OffroadMafiaS and place them in gta 3 data directory replacing the files


2 manually (if you have other car mods)
open default in OffroadMafiaS 
locate the mafia sentinel default ide line and copy it
open gta 3 default ide located in data
search for the mafia sentinel and replace it with the line from the mod
do the same thing for the handling and carcols

