Easiest way is to flip Horizon edition cars on the auction house, especially the vipers. I can make 10 million per day easily.
Seat time. Just like anything else, the more time you spend doing it, the better you’ll get at it. And get the VIP (double the wheelspin amount).
I opened my FH3 last Xmas morning, today I have almost 300 cars (most of which never get touched lol) and upwards of $70,000, with 1 Goliath race.
Get online and you’ll rack up so much so fast you won’t even realize it.
Christmas? Don’t you mean Thanksgiving?
I mean, I have close to 480 cars and most of the remaining ones are cars I have little interest in, some of which I have already sold multiple of, though I’ll probably keep a copy of each around so I can sell the duplicates I earn via wheelspins.
if you haven’t unlocked goliath just do longer races on other circuits. 10 laps of the suburbs circuit in byron bay pays about the same as 1 lap of goliath. turn off rewind, traction control and stability in difficulty and use manual gears. get a countach HE as soon as you can and rinse your favourite circuit until you unlock goliath 
you should be able to pick up a horizon edition countach for 200k in the auction house if you look for 1 that is down to 1 min with no bids. if someone outbids you let them have it and reclaim your credits and bid on the next 1 