The next few steps are probably the most difficult in this Instructable but if you think of assembling the sides as you would a cube, you should do ok.
Flip over 4 side pieces. Attach the 1x4 bricks with knobs to the pieces. Make sure that the bricks with knobs are attached to the red pieces using their extra row of studs (not the ones that are on top of Lego bricks normally) so that the normal row of studs point outwards. Also make sure that two of red pieces have the bricks with knobs attached horizontally and that the other two are attached vertically.