Easy Peasy.
During your first career rally (Croatia if starting in the Junior WRC), you'll have the opportunity to select your tyres. Make sure you take a mixture of Soft and Hard!
Now, after the first two stages you'll be able to repair your car.
Go into the Tyre Selection menu from the left, second from the top, and make sure you set the tyres up so you have, from front left to rear right:
Hard | Soft
Soft | Hard
Spares shouldn't matter.
You don't have to drive on them! The achievement unlocks when you perform the swap. So you can race the next stage on whatever you like.