You must be on the Zombies side on a Gardens & Graveyards game and you must select the "Engineer".
As an Engineer you will see a marked area with a purple wrench over top. This is where you need to go. Once you get there, hold the B button and build your portal. After each time your team clears a garden there will be another portal further up that you can build. Just rinse and repeat at least 3 times in a single game.
Note: I found the fastest way to get to the portal to build them are with the jackhammer. Just use that ability and head straight for the marked portal area!
Let me know if anything needs to be altered/changed - this is my first guide :)