This doesn't need to much of a guide, as it's pretty self explanatory. Just get a gold medal on all 60 racer events.
But, it will take time. All criteria for Gold are listed on the race intro menu.
Prepare for many retrys at certain missions (I found the time trail category to be the hardest).
*On Duels and Hot pursuits, your time does NOT matter. Just finish 1st for Gold.
* Don't always take the "shortcut". Some of them are long cuts. And what car are you in? Low slung cars go over gravel slowly. Learn the cuts and if they are faster or slower.
*If you fall behind in a duel/race, don't worry. Usually "rubber-banding" seems to come into effect, and you'll have caught up in no time.
* The biggest reason to fail a HP is being wrecked by the cops. Watch them coming up behind you, and prepare to evade/drop spikes.
* On time trials, it may take a few goes. Learn the shortcuts, and set yourself benchmarks. If you know its not going to happen, restart. The time trials are not as forgiving as the races.
* Use your power ups at the right time. Don't use them just for the 200 bounty bonus. Save them until you need them.