Despite his size, Optimus Prime unfolds from his trailer form and back again with few maneuvers. Unfolding the Generation 1 version starts at the legs and ends with the head. Reversing the steps returns him to trailer form, getting him back into the fight against the Decepticons. Autobots, roll out!
Vehicle to Robot
Separate the two main components, setting the trailer aside for now.
Pull down the now-exposed back wheels, rotating them down to form Optimus' legs. Be sure his feet are fully turned so the figure can stand on its own in its robot form.
Twist the two fender doors out and toward the front, forming the arms that'll soon wield his trusty rifle. Attach the two hands such that the thumbs point inward.
Flip up the roof door, revealing the head to swivel up. If you haven't yet, you can give Optimus his rifle while you build him a better weapon.
Rest the trailer on its wheels using its stabilizer legs with the tailgate down and extended. Pull apart the doors of the roof to bisect the piece, revealing the artillery robot within.
Raise the artillery robot, filling the empty cannons with the rockets that came with your figure. The tabs on the sides will fire their respective rockets. Remember to rotate the claw and satellite dish as well to give the robot its full arsenal.
Back to Vehicle
Fire off the rockets at the first Decepticon that looks at you wrong, then detach Optimus' hands. Set these aside.
Rotate the satellite dish and claw back into their original positions, lowering the artillery robot back into its own.
Fold the sides of the trailer back up to reshape it. Put away the stabilizer legs and replace the tailgate.
Push Optimus' head back into its small home, closing the roof door back over it.
Push the arms back like doors, folding down the fenders carefully.
Bend the feet back for compact storage while pulling the legs back, forming the rest of the hatch. You can rejoin the two components and store any extra pieces in the trailer.
You can also launch the scout car Optimus uses. Slide this back into the trailer until you hear a click. Pushing the tab launches the vehicle off in a straight direction.
Do not apply excess force; it's unnecessary and will break your older figures.