Sand all surfaces of your lumber using rough grit sandpaper, then a second time with fine-grit. Surfaces must be smooth for writing.
Set one 30-by-30 sheet and the 30-by-60 sheet together in an L-shape. The end of the smaller sheet should abut the inside face of the larger.
Glue the sheets together at the joint. Reinforce with a line of 10 evenly spaced nails. Wipe off excess glue.
Add a second 30-by-30 sheet to the opposite end of the 30-by-60 sheet. Mount, glue and nail as you did the first.
Mount the remaining 30-by-30 sheets between the two you have already mounted. Place each approximately 15 inches inward from either of the first sheets. Mount, glue and nail as you did the first two.
Stand the desk up so that it rests on the open ends of the 30-by-30 sheets.
Lay the 60-by-18 sheet along the top of the back of the desk structure. Glue in place to the ends of all five sheets. Nail in place using a line of 20 evenly spaced nails along the edge of the top of the desk. Reinforce with a line of six evenly spaced nails along the edges of each of the support legs.