Firstly - do it! It's fantastic.
There are lots of useful guides online - I did it in 5 days Inverness to Inverness which is the minimum people recommend, and itdidnt feel rushed. Between 2 and 4 hours of driving each day which is fine.
The route I did was:
Inverness to Wick
Wick to Tongue via John O Groats
Tongue to Dundonnel
Dundonnel to Melvaig
Melvaig to Inverness
Plenty of great walks, waterfalls, distillery tours, zipline, beaches etc on the way around.
Just make sure to keep topping the fuel up at any opportunity because fuel stations can just be empty when you turn up! Although I was very impressed with the GTI - around 43 mpg on the Bristol to Inverness leg and around 37mpg around the actual NC500 which included some steep inclines, spirited driving and overtaking! That's with two people and plenty of luggage, and AC on the whole time.