Start in the corner of a room --- this reduces the number of pieces you will need and helps to keep everything square and stable. Affix the 4-foot wood strips to the wall horizontally, at the height you would like your ceiling; 6 feet works well, but go higher if you are taller. Use the spirit level to make sure the strips are straight. You should end up with two horizontal strips on the wall, 6 feet from the floor, their tips touching where the two walls meet. They should form a single strip that runs around the corner, from one wall to the other.
Attach the 6-foot wooden strips to your wall vertically, running from the outer edge of each horizontal strip down to the floor. Again, use the spirit level to ensure it is straight. You should now be able to see the outline of your booth taking shape. There should be two horizontal strips where the ceiling edges will meet the wall, and two vertical ones where the sides of your booth will touch the walls.
Determine which one of your booth-sides will act as a door, and put this one aside. Stand up the other side piece against the vertical strip on the wall and hammer four of the panel pins through the edge of the board into the strip at equal distances along its length. You should now have a standing wooden side protruding from the wall of your room. It will be quite unstable, so it's best to get the roof on as quickly as possible.
Place the roof on top of the booth. It should rest on the two wooden strips you affixed to the wall. Tack it in place using the panel pins and line it up to the new wooden side you have just created. Run another wooden strip along the inside of the booth, where the ceiling meets your new wall and panel-pin it in place.
Position the final door piece and affix it to the wall strip with the hinges. Attach the last strip vertically where the closing edge of the door meets the wooden side. With everything assembled give one last check that everything is stable, and add more fixings if anything is loose. Use the glue to affix the insulation sheets to the interior, as if applying wallpaper. If you don't have enough to cover the whole inside, focus around the height of the microphone.