Hi!
Unfortunately the SD card slot is soldered on the motherboard for the 3DS XL…
This means that you need to solder it out and solder the new part in.
You do need a heat gun to get this fixed more easily. Do you have any soldering experience?
I can’t find a guide either, it won’t be easy to br honest, the power/battery connector is also located there, so you do need be very careful to get it out.