Set the arm pieces of your kit so that they are about as far apart as the futon couch will be wide. Orient them so the grooves in the side panels both face inward.
Set the front stretcher -- the long beam -- in place between the front ends of the arm pieces. Use the bolts and barrel nuts to bolt it in place. Tighten down the bolts by hand, only enough to obtain a firm grip. You will need the room from a loose attachment while inserting the mattress frame.
Repeat step two to install the rear stretcher.
Lay the seat and back portions of the mattress frame on the floor, with the inside edge of the seat deck nestled inside the protruding arms of the back deck.
Screw the sliders in position on the back deck of the mattress frame. Many models will come with these preattached.
Connect the back and seat decks with the clevis and cotter pins included in the kit. The decks should be firmly attached, but able to rotate around the pins to fold.
Slide the assembled mattress frame inside the frame you built in steps one through three, inserting the sliders into the grooves in the arm pieces.
Tighten down the bolts holding the spacers in place using the allen wrench that came with the kit.
Test the sliding action before setting your mattress in place.