When you return from your beverage break, flip the tabletop, good side down and place your trash cans, upside down, one on each end. Leave enough room between the cans for you and your chair to sit comfortably.
Trace around the trashcans making sure to mark the L brackets.
Move the cans off to the side and place the eight wooden blocks on each L bracket mark.
Nail them to the plywood top. These will raise the desktop to a comfortable sitting/ working level.
Moving On
Lower the cement blocks into the trashcans. Those heavy suckers will keep your desk from tipping over and spilling your laptop into your lap.
Have your friend help you lift the tabletop onto the trashcans, good side up. You might have to do some fancy footwork to get the wooden blocks to line up precisely on the L brackets, but the pencil outline of the trash can that you traced should help as a guide.
Once the table is in place, use the drill to screw the L brackets to the wood blocks.