Basically, when you get to your destination, you'll have three options on how to park the load: "Where do you want it?" is the choice you want to choose for this achievement. The other two will not unlock this achievement.
Your goal is to line the truck up in the outline and detach it when the game mentions it's in the right spot. It may take a few tries to line it up but unless you're doing a speedy delivery or really falling behind, you'll have enough time to get it into position. Try to get somewhat close to the outline and make small movements with the truck to get into position. Remember that when backing up, left will push your trailer right and right will do the opposite. Try to gently nudge the trailer into position with small movements, as you're more likely to lose control if you try to sweep it into position.
If you're having trouble, keep an eye out for the tough spots that will only reward you with 40 XP. These usually are much easier to park than the 90 XP ones and will be a lot easier if you're not used to handling the truck.
According to Espio Idsavant, the achievement only unlocks on 90 XP spots. Thanks to Espio Idsavant for commenting and pointing this out!