Cut one of the wooden sheets in half along the diagonal using your power saw. This will give you two triangular pieces.
Set the two square pieces up in an L-shape in the corner where you plan to install the desk. Use your level to confirm they line up. This is important because your corners are where you'll find out whether or not your floor is level.
Sand down one edge of the wooden squares if your floor isn't level. If your floor is severely off plumb, you may need to cut edges to match using your power saw.
Sand all faces of your lumber twice. Use rough grit paper for the first pass and fine grit for the second.
Apply one coat of stain to all faces of your lumber.
Arrange the two wooden squares and the wooden strip into an L-shape, with the wooden strip in the corner of the L.
Glue the L-shape together. Reinforce with a line of four nails for the edge of each wooden square. Alternate the spacing of the nails so the crossing nails don't run into each other. Wipe off excess glue and wait for it to dry.
Set the triangular piece on top of your L-shaped frame. Glue in place, then reinforce with a line of five nails along the top of each wooden square. Wipe off excess glue and wait for it to dry.
Apply a thin dab of putty over the top of any visible nail heads. Wipe off, allow to dry, then sand with fine grit paper.
Apply a second coat of wood stain to all visible surfaces of your lumber. Allow to dry, then slide into place in your corner.