Forza Horizon 5

Find the "Donut Hi" story missions, which are located just left of Playa Azul (on the right of the main Mexico map).
There are 7 chapters, each of which you'll need to complete for this achievement. For another achievement (Give It the Beans), you'll need to 3-star each chapter, so you may as well do it all in one go.

1 - Welcome to HiLow

3 Stars - 1:52
This is shared between the Hi and Low stories, so you only need to do it once.
For the most part, you'll want to just follow the highlighted track on this first mission. You can try shortcuts but the first two-thirds of the race are through heavily wooded terrain, making shortcuts high risk.
That said, when you're a little over a mile (~1.6km) away from the target, there will be a sharp right turn. You can cut the corner here, then you'll be able to ignore the highlighted path and shoot directly across open fields of agave and cempasúchil (the orange flowers) in order to hit the finish line.

I finished in roughly 1:41

2 - Welcome to Hi Team

3 Stars - 2:15
Super easy, there are no checkpoints here, so as soon as the race starts, ignore the road and turn left. Find your destination marker and beeline straight towards it over fields and rivers. The only thing to watch out for is directly next to the destination are a couple of houses that will kill your speed and momentum.

I arrived in 1:23 ish.

3 - Donut Delivery

3 Stars - 0:55
A couple of short stages in this one. The first two stages have a long target time but don't affect the number of stars you get. The first two stops require you to reach and park at a very close destination within a few minutes. This is easy enough. Drive at full speed then park at the destination (if you have the settings on to prevent any car damage, you can simply crash into the yellow Donut Media stands at each location, which counts as parking). I recommend following the highlighted path for the first two stops.
For the third stop, follow the highlighted path until you leave the city area that you're in (watch out as the first two corners are a little tight). Once you hit, the outskirts, ignore the road and zoom straight across the grass until the road straightens out once more. Just around the corner, you'll find the final Donut Media stand, which you can plough straight into for a quick park.

I screwed up the first two corners when replaying to write this guide, but still arrived in 0:53

4 - Time After Time

3 Stars - 3:00
Another straightforward one.
Start the race and follow the highlighted route as best as possible. You can cut a couple of corners but nothing major is worth the effort. Once you're through the mountain range and about 1.5 miles (~2.4 km) away from your destination, you'll see the recommended path takes you twisting and turning down a mountain road. Ignore this and simply zoom straight ahead towards the destination. There's a pile of rocks and a house in the way, but you'll have plenty of time to get around them before reaching your destination (or use rewind to adjust your course).

I hit the house but still finished in 2:08.

5 - Up To Speed

3 Stars - 2:05
This was probably the most awkward to get a 3-star on. I routinely kept finishing a second or two later than the 3-star time and it took multiple repeats. You cannot leave the track for too long otherwise you'll be automatically reset and there are checkpoints in this race, meaning you can't miss any.

To make this as easy as possible, before starting the Chapter, change your game difficulty to Tourist ( > Settings > Difficulty > Drivatar Difficulty). This places you second in the starting lineup, rather than fourth which happens on every other difficulty.

Start the race and follow the highlighted path as best as possible. You'll want to basically not take your foot off the accelerator for the first half of the race.
There is one possible shortcut. After passing through the third checkpoint, rather than following the road to the left, shoot straight forward through the vegetation and continue onwards over the savannah. You should see the next set of checkpoint flags directly ahead of you on the road. You'll get a warning from the game saying to return to the track, but that will disappear before it takes effect. Then follow the highlighted route for the rest of the race, which appears to be the fastest route.
Make sure you do slow down for the only sharp corner, otherwise you'll likely screw up the timing.

After multiple attempts, I finally managed to finish in 2:03.

6 - Cars Are Pain

3 Stars - 1:52
You're driving a lorry with your damaged car on the back this time, rather than the car itself. The lorry is pretty fast though. This is another where you simply have to follow the highlighted path and there are checkpoints for you to hit.
There aren't all that many shortcuts you can take other than trimming a little off some of the corners, but you shouldn't need to do this.

I kept my foot on the accelerator and followed the road almost exactly and still arrived in 1:48.

7 - Does More Expensive Mean More Better?

3 Stars - 1:25
This is the exact same task as Donut Low.

Another two-parter. The first stage of this sees you apply upgrades to and tune your own car. This is annoying, as while you're given a budget of 60,000 CR and a target of "A - 800", you have to hit this in a specific way.

Taken from the other guide (thanks to FatBlubfish219) and very slightly tweaked, use the following upgrades and tuning;

Press to go to upgrade menu.

Once you have completed the Tuning, PRESS TO SAVE IT. If you use to get back to the car upgrade screen, the tuning won't save and you won't ever 3-star the upcoming race.

With that all saved, back out of the upgrade screen and you should be told that the upgrade meets the requirements and is complete. Then drive a little to the right and wait in the destination highlighted. After a few seconds, you'll start a race.
Nothing special for the race, simply follow the highlighted track, hit all the checkpoints and you'll finish in time.
If on the long straight, you don't break more than around 135mph (~217 kph) then you didn't save the tuning correctly (you have to quit out the event and restart, then reapply all the upgrades and tuning).

I finished in around 1:21.

With that done, go past the screen saying you got 3 stars and as you load back into the freeroam map, you'll pop this achievement.

As always, if you place a negative vote, please let me know why so I can improve the guide.