The last pieces of wood that you need for your chair are strips as long as the sides of the table and about 2cm (2.78 inches) wide. You will need these to keep the back and arm pieces fixed to the seat. Cut the edges as a half triangle so that they can easily face each other (see pictures). You'll need 5 of these.
Now make 3 holes through them and attach them to the back pieces and arm pieces using screws, right at their bottom, on the same side where the foam is. And for the longer back piece, you need to place it near the other 2 pieces to know exactly where you have to attach the wood strip.