Steps, page one:
As above, measure and drill 15 holes in both pieces of plywood with the forstener bit or spade bit. I spaced them 110mm apart (from the centre of each hole) and in the centre of the 60mm plywood, marking the centre for each hole with a cross. That’s a lot of centres. I made the holes 9mm deep.
Chamfer all edges (or get rid of the sharp edges) with the 150grit sandpaper and take the burr (scratchy bit) off the holes with the 240grit paper. Check the fit.
On the broom handles, measure 450mm and 900mm along the length. Mark with a pencil, dividing the handles in three.
Cut with your handsaw. Don’t worry too much if they aren’t exactly the same length but try to get them close. They are now your dowels.
Smooth the sawcut edges with the 240grit sandpaper.