Halo Wars

Spartans can "jack" enemy vehicles by pressing Y on them. Simply press Y on a covenant vehicle, and he will take it over after a couple of seconds.

However, you only have three spartans and you need to capture 6. Just get your spartans out of the vehicle by getting the vehicle destroyed.

Easiest done on Normal. On Easy, your army will usually kill everything before your Spartans have time to jack it.

This a good video showing how to get the achievement.

*This is not my video, all credit goes to RoosterTeeth*

I just wanted to put forth the strategy that I used.
NOTE: This can be done in co-op, so if you have a friend that will play with you, it can be a lot easier as you can each control a spartan at the same time.

Make sure you play on normal or heroic so that your spartans vehicles will die without killing everything that comes at them.

Since you need to jack 6 vehicles and you only have 3 spartans, it is imperitive to have the vehicles die. For this reason, I suggest trying to jack ghosts. They have lower health and less power than a wraith and will thus die easier.

Try to keep only a few men at the mouth of the pass to defend your base, but be sure to keep your spartans on the frontlines. I suggest Alice and the rocket spartan (Thomas?) as they don't do as much damage to vehicles as Jerome.

Once you capture a ghost, keep it farther ahead so that it will be the first thing that get attacked. Once it does, keep it in the fight until it is almost dead, then slowly back up. If you time it right, the ghost will blow up, but the spartan should be able to recieve support from your main forces. Rinse and repeat five more times.

*On a side note, once you have about 3-4 jacks I suggest jacking wraiths. Once you do however, keep them near your base and out of the fight. That way, when it comes time to finish the level, you can have an army of wraiths along for the ride.