F.R.I.M. is "Free Roam Instant Money", which allows you to gain a small amount of money while in Free Roam by either "Dodging" (Near missing cars while driving above a minimum speed), "Drifting" and "Jumping" (Driving at speed over a bump in the road, the higher the air you get, the more reward). It appears at the top of your screen and you can progress through the 10 levels, each of which you can "Bank" that levels amount of money. With each level increased you'll get a higher reward.
Before you upgrade your FRIM, level 10 offers $4,000. After the upgrade, which is granted at Level 12 Competition & A1 License obtained (unconfirmed) the Level 10 reward is $10,000.
Due to the upgrade offering a substantial amount more than pre-upgrade, I'd suggest waiting until that happens before going for the $100,000. It'll save some serious amount of time.
There are 2 main methods - One is safe, the other is risky but a lot faster.
Method 1: Drifting - Find a patch of road/land that is farely secure and out of the way of other vehicles or obstacles.
The best place to Drift risk free is in the Airport. Once there, you can drift around the figure of 8 safely and build your FRIM til you hit the maximum level.
To drift, put your foot to the floor by pulling and hold for 1 second while moving the left or right. Make sure and hold the the whole time and try and keep the slide as wide as possible. Keep repeating the Maximum level to get the $100,000 also. This is very time consuming and if you feel you can control your car, I'd suggest Method 2.
Here is how to get into the Airport:
Method 2: Dodging - The best car in my opinion to use for this is the Ariel Atom. It is a fast car that is extremely easy to handle. The best stretch of road to use is the road heading East from the Airport in Oahu, Hawaii.
When you exit the Airport, take a right going East and floor it, try to keep in the middle of the road dodging the oncoming cars (The reason it's the quickest way is by dodging oncoming traffic, this builds your FRIM meter faster than anything else) and cars going in your direction. This stretch of road is possibly the longest, flattest road in the game. It gives you plenty of time to get the full level 10 FRIM by the end.
Once you've either hit the end, or you've hit level 10 - Just turn around and repeat until this achievement pops!
Big Tip: I'd suggest pulling out your Ethernet Cable/Wireless Adapter and doing this offline (both methods) as it will ensure you don't get any wreckless drivers colliding into you while you've built up a massive streak and wasting even more time.
Go to the Airport in Ibiza Area 2 (south end of the island) Just east of there there is a small parking lot looking area , you can drive in easily and its empty you can do laps uninterrupted just drift the corners. It took me about 3 minutes In a decent car
This also works as a great area for the Level 10 F.R.I.M.