Use a drill or a rotary tool (Dremel or similar) to cut a starting point for a coping saw or jigsaw. Insert the blade of the saw through the hole you made and cut the window opening. (make the hole just back from one of the window corners, and cut to the corner to make it square.) Use a file and a sanding block to make sure the corners are square.
Cut out the opening for the door and square the top corners the same way. You can cut your door opening directly from the base of the side piece of wood. You do not need to leave a narrow strip of plywood along the bottom of the door.
Test fit the window. If the opening is too large, you can shim the window into place from the wrong side. The trim will cover the shims.
Test fit the door and make sure it swings through the opening, leaving enough space below the door to fit the flooring.
Correct the fit if necessary
Check the door trim Check the thickness of the door and window trim while you are fitting them to make sure they do not push too far into the room. You may need to trim back the unfinished side of door and window casings so that they are not too thick for the ¼ inch Baltic birch walls. The amount you trim back may need to be adjusted for the thickness of your wall coverings.
Plan Wiring Runs If you want to have working lights in your roombox, plan how the wires will run. Will they come out the top of the box, run up the walls or along the baseboards? If you will be covering tape wire runs by putting your wallpaper on a piece of card to install it, you may need to leave a wider trim on your unfinished window and door casings to allow for the extra wall thickness.
