For new players: Don't worry too much about this one. It'll probably unlock along the way while completing the base game and dlc's. Ultimate skills will come naturaly by just racing and exploring the open world. If not, i'll mention a boosting method below.
The amount is shared between both races and skills so you don't need to focus on a single task. I had about 700 completed races and 6000+ ultimate skills (too many Forzathons) when this popped retroactively for me. I wasn't even playing, just checked the achievement list through the dashboard and they unlocked.
Mind you, ultimates skills are divided across drifts, air, speed, burnout, etc, but they all count torwards the achievement. You can check by pausing the game and selecting the "stats" screen located in the "progress" tab.
If you need to boost it, i'd recommend going to the desert area (close to the airport) with a fast offroad or rally car (or anything fast with offroad tires and suspension) and farm ultimate air skills. They're super easy to get (about 2 or 3 seconds of air time is enough) and there's plenty of dunes to string jumps together.
If you need both this and the "Hotwheels skills" then do the later first by farming on the Hotwheel's Thrilltopia map. "Hotwheels Skills" are one step further from "Ultimate", so you'll be working torwards both at the same time. Check it below for a guide with the best method