The best place to do this is in the early stages of campaign.
Choose your car carefully: you want one with
high strength, and the
aggression style of boost. If you're new to the game, your best cars are likely to be in the 'Legendary', 'Cops and Robbers', or 'Paradise Island' categories, because they're all unlocked from the start.
Find a Road Rage event - they're red blobs on the map - and play safe until you have 10 takedowns in a row.
With the aggression style of boost, your bar refills with every takedown, so you should have full boost almost all of the time. That doesn't mean you should hold it down the whole time, though! Use it strategically: boost to catch other cars or to bump them in the back corner, knocking them into a side wall. It won't take much to take them out; just a helpful, boost-assisted love tap.
If you get taken down, you can restart the race by pressing the D-Pad right, then right again
.
PS - there's an easy Road Rage event near your original garage which will let you take to the freeway for an easy drive. Try to hold the centre of your side of the road, so you can easily nudge opponents into the side.