This is how I got most of my skill points, just go to the location on the map from the video and continuously drive around the small patch of grape vines jumping and sliding around keeping your combo until you want to cash in your points. Keeping your combo going is really easy in this location!
Obviously having more skill perks that are related to building skill points will help you build bigger combos. I used a car that was a "Horizon Edition" with a Skills Boost which lets your combo multiply go higher than normal, but it doesn't affect your maximum multiplier while a skill song is active.
Also for the possibility of longer skill songs, change your radio station to "Timeless", you can get up to a 23 minute skill song
[Edit]: You can only get a max of 3 skill points per combo which equals to a 300,000 point combo, so if you want to gain skill points id recommend stopping your combo at that point, then start again to get the most skill points in the least amount of time!
Thanks to Tendoman77 for pointing that out