Step 7: mattress and upholstery
Mattress: If you did not buy a ready made mattress for the crib, you have some sewing work ahead of you. Buy a waterproof fabric (about 100x100cm) and make a case for the foam mattress. I recommend also making a removable sheet that you can take off and wash so the baby wont be directly on the waterproof material which is usually not comfortable.
I don't have any pictures of making the cribs mattress because I bought a ready made mattress that was twice the width and just cut out half the foam from inside.
Seat: Cut a piece of wood 42x44cm (at least 0.6 cm thick), round out the edges, glue the foam mattress to the wood with super glue (or any strong glue), cut the edge of the foam at about a 45 degree angle to give it a slope at the ends. Using a staple gun, tightly staple the fabric of your choosing (I chose black fake leather) from all sides to the bottom of the wood then cut off any extra fabric. Center the entire thing on the seat and secure it into place using slightly long screw from the bottom (make sure your screws are long enough to get through the base but not through the wood you used for the upholstery)