Hello,
I tried this method and created an emuNAND, but my problem is that I don’t know how to boot nand from a microSD cart. I had a look at the discussion chain from MathewPKfan2001 but I didn’t understand anything :/
Do I have to solder again, to use this microSD cart method?
No on the soldering part. In order to boot EmuNAND, you would have to be able to boot to Luma3DS configuration when holding the (Select) button and selecting the (x) Autoboot EmuNAND option. This won't work if your n3DS motherboard indeed has a hardbrick.