Gather your supplies
2.4 x 1.2m x 19mm MDF, cut in half; 19 x 19mm x 1.2m pine-edge battens (2); 1.2m-length piano hinge; 70 x 35 x 900mm pine locating battens (2); 42 x 19 x 100mm pine lock blocks (2)

NOTE: The measurements for this project suit a 1500 x 900mm six-seater dining table. The length of the locating battens should be the same as the width of the original table.

Here's how
Step 1 If MDF is not supplied cut in half, use a power saw along a straight edge, to form two 1.2m square panels. Apply glue to a cut edge and smooth out so it will give a full glue joint.

Step 2 Cut, predrill and countersink 1 edge batten with 5 screw holes. Align with the edge of the tabletop. Screw on the batten with 50mm countersunk screws after predrilling the edge of the MDF to avoid splitting or bulging the board. Glue and screw on the other batten.

Step 3 Screw the piano hinge to 1 edge batten so that the centre of the hinge barrel is in line with the tabletop. Clamp the 2 tabletops together, top surface to top surface, then screw the hinge to the edge batten of the other tabletop.

Step 4 Put the tabletop on your dining table. Make sure it's centred in all directions. Hold 1 locating batten in place at the end of the original table, predrill, then glue and screw on from the top. Repeat for other end, but don't make the fit too tight.

Step 5 Turn the tabletop over and screw on a small lock block at the centre of each locating batten. Screw on so there is some friction when you turn the lock block.

Step 6 To use, lift the folded panels onto the table. If your table is polished, put an old tablecloth on it first. Open up the leaf and turn the locks to hold in place. When set up, a small ridge remains at the hinge, but you should be able to work around that when you set the table.