You will need basic woodworking skills and access to common woodworking tools.
Bertil Chair and Ikea supplied fixings/hardware
1200mm x 400mm x 22 mm plywood sheet
1200mm x 45mm x 20mm pine
1200mm x 20 x 20 pine
Scrap bits of pine
PVA wood glue
Step 1: Halve the chair
Work out which rails need to be cut in half.
Measure them to find the centre.
Saw them with your preferred timber saw.