This is much easier if you have the Hot Wheels DLC, but will cover both ways.
If you do not have the DLC a 'Stuntman' skill is when you get air, land, and almost immediately get a 'Near Miss.' One of the easiest spots to do this solo is near the Redstone Gold Mine, around the path at the bottom of the refinery plant where there are plenty of hills to get air that are near a road. This is also significantly easier with an online partner who can stay still but if you are the solo type like me I show one way to get the 'Stuntman' skills.
'Hot Wheels Air' consists of basically hitting the jumps on the car tracks in the Hot Wheels DLC, with or without the speed boosts. If you are trying to drive down every road in the DLC you will most likely get this naturally.
Both skills will count together for the 20 as I had 2 'Stuntman' and 18 'Hot Wheels Air' skills when the challenge popped. Also, you do not have to bank the skill chain for the skill to count toward the challenge, as shown.
Video below for reference on both the 'Stuntman' and 'Hot Wheels Air' skills.