This is tricky! I used my finger nail to open it since I was scared to put too much strenght with a spudger. Be really careful when moving the retaining flap. Just open it 90°, no more or you’ll breack it!
Putting back the ribbon of the (new) touch screen was the hardest part. The ribbon was bent to the side and my fingers too big to grab it withouth bumping on the mother board.
I finally found that holding the mobo with my left hand, looking at the ribbon from behind and pushing it with my index was more convenient, although my index was bumping on the retaining flap, thus partially closing it. After many trials and sweat, I finally inserted it and closed the flap! But the ribbon detached from te slot! :’( I put it back easily this time (after reopening the flap of course). And closed the retaining flap, pushing a bit harder this time, making sure I could feel a “click” confirming the proper closing. Don’t put too much strenght on it though!
The slot to insert it is so tiny! Use a magnifying glass if you can.
After my first reassembly, the top screen was not working (bottom screen briefly flashind and DS turning off imediatly after swithcing on).
I found out it is the top screen that is not connected correctly.
You have to insert the ribbon until the part which is wider than the rest of the ribbon, almost bumps on the white connector.
This was too difficult with just my (too big) fingers. I used the spudger to push the ribbon from behind and maintain it when closing the retaining flap. This is kinda tricky, but it worked!
When reassembling I’d suggest to pass the antenna (black wire) between the charging port and the small EM10 chip.
I first put this wire under the EM10, I couldnt close the case of the DS. There was a resistance near this spot. I gues it was the antenna wire.
Although, to put the wire under the cartridge slot, under the EM10 is the best spot. If you look under the catrige slot, you can see there is no obstacle there.
I got this wire stuck under the cartrige slot when reassembling because it was bent upward when I passed it under the slot. So it was bumping against the top of the cartrige underneath sapce. So my advice is to bend it to get it straight before feeding it through the slot. Attach it to a thread before removing it, as advised in other comments, is a good idea too. Just secure the first knot by a second one or it might detach when you pull it.