Sand three 80-inch-long, 2-by-10 planks. These are the support planks that will go underneath the platform of your extra long twin bed. Sand a 44-by-84 inch sheet of 3/4-inch plywood; this will be the platform of the bed.
Apply wood finish to the support planks and platform. Applying the finish, before constructing the platform bed, is much simpler than doing so after the bed is assembled.
Stand the three support planks up on a 2-inch edge and 15 inches apart with their 80-inch sides parallel to each other.
Center the platform over the top of the three planks. There should be a 4-inch overlap on all sides.
Secure the platform to the three support planks with 1-inch wood screws driven every 5 to 6 inches through the platform and into the top of all three support planks.