Surefire way to get this - Find a buddy and have both of you create a blueprint. To create a Blueprint, go to your Career mode. On that menu, go down to Cars (3rd Option), and either press right on the D-Pad or hit A. Go down to Garage and hit A.
Now here, you just need to pick a car and go down to Create New Blueprint. Tweak it anyway you'd like, which includes purchasing parts, and hit B to back out. It will prompt you to save, do this.
Now go ALL the way back out to the main menu. Go down to Share, and find your buddy's Gamertag. Hit A, and go down to Share, hit A again, and hit A on Share Blueprint. Pick the Blueprint you just created, and send it.
Make sure your buddy downloads it, or the achievement will not unlock. After you are sure your buddy downloaded it, go back out to the Share menu. Hit A again, and hit the Right Trigger. The achievement should pop then.