For best efficiency, do this at the same time you work on the three tuner achievements. This is easiest to do with a second profile on your Xbox One, but you can also do it with a group of 15-20 other people where you each commit to a 2-3 week boosting process consuming about 20 minutes a day. This guide is written based on doing this solo with a second profile.
On your main profile, you will need to create and share a paint job for one of the cars initially given at the start of the career. To do this, go to the Career menu, choose Car, and get in the vehicle in question. This will place you back at the Career menu, so choose Car and then Paint. You can apply decals, create vinyl groups, etc. The fastest approach is merely to paint the car all one color, then press
and choose "Apply to the car and save". In the menu across the top of the screen, scroll over to "My Designs" and press
. Your current car and design should be selected, so press
and choose "Share". Choose two keywords (mandatory, but which ones don't matter) and then any description you wish, then choose "Share". At this point, your design is able to be downloaded and used by others in the community.
Now switch to your second profile. If you haven't created one on your Xbox One previously, you need an email address to do so, and Microsoft will give you one at no charge if you go to outlook.com. When you are signed in to your second profile you will need to load up Forza 5. Note that if you have never played Forza 5 on the second profile before, you will need to progress slightly in the campaign before it will let you exit to the main menu. When you are selecting your initial vehicle, pick the same car as you chose above on your main profile.
Once you can get to the main menu with your second profile, go to Career, Car, Paint, and choose "Find New Designs". Press the search button and under Creator put in your main Gamertag and choose "Search". The paint job you just created will appear, so press
to select it and apply it to the car.
From the main Forza screen, choose Multiplayer, press
to create a private match, and
to confirm. Once the vehicle selection screen opens, ensure that the car with the downloaded design is selected. At this point, you will be in the lobby. Press
to change the options to match these choices:
Game Type: Drag
Track: 2-player 1/4-mile drag
Max Players: 2
Max Drivatars: 1
Drivatar Difficulty: New Racer
Heats: 10
Car Class: Any
When you go back to the lobby, press
to start the race. If you press
as soon as the Post-Race screen appears, t will take approximately 4-5 minutes to complete the 10 heats, at which point you will be back in the lobby. Press
to restart the process. You will need to complete a minimum of 100 heats and a maximum of 150 heats on each day. If you complete fewer than 100, you will get credit for zero. If you complete more than 150, you will get credit for 150. The official day for Forza 5 is based on the time zone for Turn 10, so it restarts at midnight GMT. You will need 315 heats to earn all of the design achievements, so the most efficient approach is to spread it over three days with 110 races each day.
In-game notifications of the credits earned should arrive around 1 am GMT in the Message Center. From the main menu, choose Message Center and press
twice. A message titled "Design Award" is what you are looking for and will list how much money you made from designs the previous day. Any achievements will unlock upon reading the message.