Many contractors will tell you that pouring concrete over wood can be a tough job to undertake. For example, plywood is notoriously difficult to pour over since the wood is very porous and won't allow the concrete to settle evenly. However, concrete-like mixtures exist that mimic concrete's appearance and durability and may provide the solution. You can find these materials at many home improvement and hardware stores that sell concrete products.
Remove any debris from underneath the plywood, if possible. The ground needs to be as level as possible, so after making the under surface flat, place a level horizontally across the ground to ensure it is level. Lay the plywood over the floor and use the level again -- if you find unevenness, you may have a warped piece of wood, so you'll need to replace the piece before commencing the pour.
Fasten down the boards to prevent shifting during the pour. If they are already secure, move on to the next step; if not, consider purchasing wood-like plank braces from a company such as Hardiplank. Screw the braces over the plywood as directed.
Apply strips of mesh tape over the borders of the plywood to better seal them together into one large, cohesive piece of wood. Mesh tape is useful in drywall jobs since it's porous and lies flatter than other tapes yet still maintains adhesiveness in moist conditions.
Mix a small batch of quick drying cement per the manufacturer's guidelines listed on the packaging. Shovel a trowel-full of mix over the tape that covers each plywood seam. Smooth out a thin layer of concrete mix until it is flat.
Spray primer over the prepared plywood surface using a paint or garden sprayer until you've covered all areas. Allow a few hours for the primer to air dry.
Mix the concrete topping as directed and begin the job by applying the coating at the furthest point away from the door. Work your way from this point toward the door until you've coated all wooden areas evenly. Smooth over the topping with a pool trowel until smooth.
Allow a full day for the coating to cure completely before attempting to stain or paint over the solid topping.