Forza Horizon

First off, I suggest upgrading a Subaru Impreza WRX STI to B500, although the Mitsubishi that you begin with will also do the trick. Now select the Beaumont Rally Series, which is the first one available.

Before the race, select difficulty and make sure, you have the following settings:
Shifting: Manual
Driving Line: Off
Opponent Difficulty: Hard

Now, I suggest aiming for this achievement on Beaumont - Stage 2, as the it is short and the CPU doesnt put in a very good time. Keep in mind, you should feel free to use rewinds. You will only need to come in first in 1 stage, and this is by far the easiest.

Video below for additional information.

Hello,

These are already some good guides for this achievement, but if you were struggling like I was on the Beaumont stage 2, then I would suggest trying the North Plains stage 1 - although it's a night stage, there is very little gear changing required, you can pretty much stay in 4th and 5th for most of the race but just watch out for the last section of the stage due to the winding bends.

I just made sure I was about a second in front at each check point and if I slowed down or dropped a few positions, I rewound until I crossed the check point ahead of the rivals then went for it.

If this helps at least one person, then I'll be happy bunny.

P.s don't forget to select your settings:
Before the race, select difficulty and make sure, you have the following settings:
Shifting: Manual
Driving Line: Off
Opponent Difficulty: Hard

There is not as simple and clear as it seems from the first time :)
You need to turn OFF BOTH automatic shifting AND the Driving Line and then WIN one stage. Other parameters you can turn ON or OFF relying on your driving skill.
Best way to win this is a first stage of Kettle Hills Rally with Subaru Impreza WRX Sti.

Good luck and have fun ;)

I did this on Beaumont rally stage 2. I'm not going to give an exact tutorial on how to do it but use a class B500 Subaru Impreza WRX STI.
The thing to do is just go hard at it, and not worry about crashing because you can rewind and tweak your speeds a bit on each corner, this may take a few runs but you will hit it eventually.

Some places you are going to be going fast and need to slow down quickly, always drop at least 2 gears on the hard corners and put your finger straight back on the throttle afterwards.
After breaking or anything, drop a gear and start accelerating instantly for the best results and to prevent losing much speed.