This part is a bit tricky. Too hard for a little kid to tackle, but still easier than an Ikea closet.
Take one of the 14-cm wide seat boards and lay it on a flat surface. Measure and draw a line parallel to the edge 5 centimeters from the edge. This is your guide for where the cleats are going to start.
Take your cleats and lay them on a flat surface, their top edges all lined up. I used the table edge to line them up. Lay your board on top of the cleats. The line you drew will line up with the top edges of the cleats. Be patient. This is a little bit tricky.
Take a ruler and draw a line on the board to use as a guide for where to put your screw marks. Estimate the center of the cleat and make a line up the board for each cleat. Now you can use your screw template to mark where the screw should sit.
Drill and screw in one screw for each cleat on this first board. Now the hard part is over.