Yeah, sorry... My answers are loooooooooong and often very complex, always rambling going in to places there were not suppose to go but hey i have been looking for those socks and sniper v2 is free on steam ihavetodownloaditbutfirst... you get my drift.. Also i tend to edit heavily often adding whole paragraphs and it gets messy..
The desktop folder is just a storage, place to keep mods that are not needed.. PrestoGP uses F3000 which is default, stock race 07 content. Most vehicle mods can be put to steam\steamapps\Race 07. Good way is to just unpack them to you mod storage, then move it to Race 07 root folder.
If you know how to make screenshots, that can be helpful as we can see exactly where the problem is. If you want to learn modding, you have to learn how to share screenshots, trust me on this
Simply just fastest way to describe a problem...
About modding in general, check your Race 07 folders. Browse thru all of them, get familiar with it since that is the first step. Here's, i hope, short descriptions
GameData: has all the GameData, vehicles, models, textures, sounds, UI elements.. Everything. Inside GameData you have:
\Teams: this is the vehicle data. Models, textures, sounds and the thing you are going to touch first, configuration files.
\Locations: track data, models, textures and configuration files (sounds can be there too but it's rare). If you have a little bit to spend, buy Bobs TrackBuilder. This is my main program as so far, i have only released tracks and i'm tutoring a quite large and active community of builders, just to pay back all the help i had when i started. BTB is good tool for modders as that allow you to make test tracks like 20km straights, 5 degree bends, skidpads, places to really test the physics of vehicles to remove opinion and "i fell like more grip".. You get pure telemetry readings that can't lie. But if you don't want to, i can make you what you need. I actually like making those
\UIData: All the icons that relate to tracks or vehicles. There is a another UIData folder at Race07 root where everything you see when in any menu. Also start videos, we have our own PrestoGP themed start videos and lightly customized game menus (someone find that link, id did it but can't locate it now....)
\Sounds: All the sounds. Vehicles use them, also on track those horns etc. you hear are in this folder.
\Talent: AI drivers and their talent files. You may need these but only at the laters stages, just before you release a mod. i don't even know how they actually work, i just copy&edit&paste.. So far i have done excactly one talent file, ever.. That because i give a rats ass about AI in this game, on my tracks they can do comparable laptimes, that's it.
First bits of editing are done in configuration files but i wont go in to these yet. Good place to start is to download a single car mod, something that is very small in size, has maybe few drivers and only single car. Open files in Notepad, read them, try some edits, experiment, delete when it eventually becomes so messy that game wont start... That is how 100% of modders get their start.. When you get this far, i'll give you the Modding Bible. Before you learn a bit on your own, that will not help.. You are fortunate in the sense that there are very few people in this planet that knows more about this game and i'm still learning.. It seems daunting but is actually very simple.
David is right, my posts are too long, at least this time i wont edit.. maybe typös...