1.) Open up IMG Tool.
2.) Find the car or object you are going to replace. For example, the Banshee. So, you find the banshee.txd amd banshee.dff files. Just press 'Edit' and press 'find'. Then, type in something like 'Banshee' to find the files.
3.) Ok, once you've found the files, select them both and extract them, you can do this by clicking the right mouse button. The option should then appear (I'd suggest at this point creating an IMG Tool backup folder, for all backups). So, extract them to that folder.
4.) Next, on both files, the txd. and dff., hit 'Commands', and then 'Replace'. Replace each files with the respective ones that came with the mod.
5.) 'Commands' > 'Rebuild Archive'.
That should do it! Go and test them. If it did not work, or you still have concerns or queries, don't hesitate to ask me via MSN or PM.
If you still don't feel really confident, you can always use SAMI. Really simple and easy to use.