Great guide, it could also do with how to replace the rubber top half of the circle pad too. I've also done this. From the photo in step 8, there's a thin split plastic washer underneath the underside of the circle pad still attached to the 3DS. You can carefully slide this washer out through it's split, then you can remove the upper half of the circle pad by carefully sliding one prong of the C shape through the hole, sliding it through going round, then the other prong will side out and it will be removed. To replace, do it in reverse, but it's very fiddly sliding the split washer back into place, but it can be done with tweezers. On all 3DSs I've opened, the notch on the C shape on the underside is facing down towards the bottom, on this photo it's shown facing the top, but I don't know if it makes any difference. This applies to both old and new 3DS XL and I imagine applies to others too.

It would be good to show photos how to carefully peel off the membrane, so that the contacts can be cleaned, or replaced. This is shown in white on the PCB on photo 11. I put further details further down where someone asked a question. This applies to other 3DS models too

I had the same problem, the A button especially was very temperamental and had to push it very hard to trigger. I watched a video yesterday which showed carefully peeling the transparent membrane off with tweezers to get inside the metal domes (it's very sticky so was difficult to get it to start peeling, but comes away carefully once you manage to get it started, just don't peel too hard that it rips). This is what holds the 4 metal domes in place. I then cleaned them as there was slight corrosion in there. I then pushed the membrane down and reassembled where it now works perfectly. I then did the same with the D-pad which worked but didn't always trigger when it clicked but would it you pushed them slightly harder and they are working perfectly.

One tip, if possible, don't completely remove the transparent membrane. just peel it enough to gain access, so you can push back down .if you peel it completely you'll have to realign it perfectly, and if it doesn't line up they won't work. The tolerance is tight!