Assemble the 2 headboards and set aside. You’ll come back to it later.
Next, you need to resize the length of the second bed rails to fit beneath the top FJELLSE bed. It should be long enough the contain your mattress, yet short enough to slot under easily. Join the two bed rails together with the two leftover pieces of the footboard.
Then, shorten the wood slats of the second bed too, so the trundle bed fits neatly beneath the top bunk. Secure the wood slats to the bed rails.
Once your trundle is done, you can move on to the top bunk.
You’ll need to measure how high the top bed should be from the floor, making space for the trundle and mattress to slide in.
Mark the new placement and drill new screw holes for the FJELLSE hardware. Then attach the headboards, one at the bed head and the other at the end.
Place the FJELLSE wood slats on the top bunk. Slot in your trundle bed and you’re done.