The objective was to build a table from a 6ft x 4ft plywood sheet and only use that material. 
The size of the table - 3ft x 4ft. 

1) Top surface - 3ft x 4ft - 122mm x 91mm  - 1no
2) Legs - 15mm x 80mm - 4nos
3) I beam - 15mm x 72mm - 1no
4) Base board - 15mm x 45.5 mm - 2nos

This is simple. You should be having a 4ft x 6ft , 19mm (or 16mm) board. 

Cut the board into two 3ft x 4ft boards. Pretty straight forward cut. 

Out of the board that is remaining, cut out the 4 legs for the table. 
Dimensions of the legs to be cut - 15mm x 80mm. 

Use the angle marker to mark the edges of the board. This way you will be able to make the necessary cut. 

Keep them like an X mark with the following specs and mark the positions. Drill 4 holes on the top deck. Apply wood glue between the two legs and bolt them into position. We used 40mm screws. Since the boards are 19mm, with the screw going throw two 19mm sheets, a little bit of the screw tip protrudes out of the sheet. You might want to use 35 mm screws. 

Similarly make the other leg of the table.

Cut of the right size of board. !

Cut of the right dimensions and thats it. 

Place a set of legs on a base board, alight to the center of the board and mark the outline of the legs. 
Drill holes on the base board. Make sure that the bolt holes are not too close to the outside. This is because since the legs have an angle, if you screw close to the edges, the edge of the screw will resurface.

This assembly becomes the entire base of the table. Its everything in the table minus the top deck. 

The I beam need to be placed vertically so as to prevent the table from wobbling.
Attach the ibeam to both the legs at the center
 

Screw the deck onto the base assembly and thats it you are done.