Always start from one end of the timber, by making a mark ,
mesure the length of the 8foot stock making 2foot centres from marked end 2foot 4foot 6foot,
with both ends of the 8foot timber mark off the edge about 3cm in, then drill two holes under each other about 2-3cm apart.
Then with 5 4foot timber strips line them up on the drilled ends and in the centre of the 2foot 4foot and 6foot markings, screwing them together on markings making sure you start at the marked end,
this is so that the frame stays strait all the way around.
Then with pva glue over the top of the timber frame when lay the sheet of wood over it making sure all the corners are flush as you can get, then with a nail gun go along the top of the wood then down the left side of the frame to make an L shape, then to the right then the bottom of the out side of the frame,
Then mark the 2foot 4foot and 6foot centres of the frame by marking a line across the frame were the screws match up then nail across each line,
making the nails about 4inches from each other, and then make the floor end.