This 'able' is a Ikea Bertil chair hack/conversion that I have been wanting to do for a while. This is not my idea so to give credit where it is due: I found the idea on Pinterest which lead me to the original from a company of great up-cyclers: Junktion out of Tel Aviv.
You will need basic woodworking skills and access to common woodworking tools.

Materials:
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
Wood screws
Paint
Tools:
Jig Saw
Cordless drill
Tape measure
Screwdriver
'F' Clamps

Step 1: Halve the Chair

First step is to gather all the parts of the Bertil chair, lay them out on a bench and cast your eye over the Ikea assembly instructions.
Work out which rails need to be cut in half.
Measure them to find the centre.
Mark out.
Check!
Saw them with your preferred timber saw.

Step 2: Assemble the Two Halves

Layout the two halves of the Bertil chair first, then assemble using the Ikea hardware.
I glued all the joints to provide extra strength.

Step 3: Mark and Cut the Seat

I started by cutting a seat 1200mm x 400 from 22mm thick plywood.
1200mm (or about 4 feet) provides seating for 3 people and fits my dining table.
I used the Bertil seat as a template to trace the ends of the bench seat. The ends are angled back, are notched around the back of the chair and the corners are rounded.
Cut with a jig saw. (tip: flip the ply over so that the top will not chip during cutting)
Sand the edges down.

Step 4: Glue'n'Screw

Apply plenty of PVA to your 2 chair ends and clamp them in place to the underside of your bench seat.
Use the provided brackets to attach.
I added some pine mounting blocks to the ends for extra strength. Just glued and screwed.
Remember to pre-drill pilot holes so the pine blocks will not split.

Step 5: Under Bench Rails

To keep the whole bench from collapsing, I added a large rail running along the underside of bench and a smaller rail from one chair half to the other.
Just glued, screwed, clamped and while clamped, pine mounting blocks attached.