Before I have time to put together a detailed set of instructions with images, this will have to do.
First, figure out what’s what. There are two sides. They are the same, left and right. The sides have a top and a bottom though. Along the long side, on the top it has shorter notches than on the bottom. This is because the bottom of the case is thicker than the front. The top of the case has holes for the jacks. The other long side is matching without the holes. These pieces have shorter notches on top just like the sides for the same reasons. You’ll have to remove all the brown paper before you start putting it together. All the screws go in the same way. You slide the nut into the slot and stick the screw through the hole and thread it in. You have to kind of hold the nut in while you try to get the screw to thread. I screw all the sides together and to the bottom to make a 5 sided box and have all those screws in before putting the synth in. Once you get that topless box built, put the synth in and insert the large screws from the bottom into the standoffs. Once you get it in, putting the top on is the final step. These nuts are the hardest because you can’t hold them from both sides. If they fall in, you can shake them out through the hole for the MIDI jack. Sometimes, the sides have to be loosened a little because they might lean in and prevent the top from sitting flush.