New Wallpaper Achievement
Get 50 downloads across all of your shared photos.
To capture a photo, you can pause a race at any time and choose Photo Mode in the menu at the top of the screen. You can move the camera around as you wish and press to take a picture. For this achievement, I suggest taking 5-10 pictures. When you take the picture you will then need to press to save it and then to confirm that you are saving a new picture. You will be prompted for a unique file name and the share menu will appear. The two keywords are mandatory, and choose any text you wish to use, then choose "Share". At this point you can continue taking pictures or press to exit. When you are done with the race, exit out of the game and switch to a 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.
From the main menu choose "Forza Gallery" and then select Search. Enter in the gamertag from your main profile under Creator, and you should see the 5-10 pictures you uploaded above. Press to download a picture, to go back to the picture list, and right on the to choose the next picture. When all of them have been downloaded, press until you are at the Forza game screen with the picture of the McLaren P1, sign out of the secondary account, and sign back in to your main profile. Now that each picture has been downloaded once by someone else, you can now download them yourself and have those downloads count. The most important detail is to back all the way out to the McLaren screen after downloading them. You can look at the progress bar on the bottom of the achievement in the Achievement App to track your progress, or merely keep going until the achievement notification pops up at the bottom of your screen.