To complete the Forza 5 achievement smooth Silver you need to change your badge to Silver Badge that you have won by completing in game challenges Once you have put on your badge you will be able to complete these challenges easily Easiest way to complete the Smooth Silver Challenge is below
Bronzed
10 Cars In Your Garage Or Reach Level 5
Smooth Silver
Reach Level 15, Drive 250 Miles Or Win 50 Races
Digging Gold
Complete 4 Career Volumes ,Drive 500 Miles or Reach Driver Level 30
Going Platinum
Drive 1000 Miles Or reach Tier 8 in Forza 6
All These Achievements are completed the same way