2DS NAND pinout:
https://3dbrew.org/wiki/Hardware#2DS
MicroSD to SD pinout:
http://elasticsheep.com/wp-content/uploads/2010/01/sd-card-pinout.png
Just make sure you keep your wires short, and use DAT0, not DAT1/DAT2/DAT3 (if you are going to use them to speed up write/read speed, I believe you must use all of them. Otherwise, only use DAT0, CLK, CMD, and GND (VSS).)
I'm not sure how much experience you have with soldering (probably more than me) but just remember:
- Use more solder to get solder off: Don't heat the motherboard too much or you may cause permanent damage to it
- Use soldering flux to prevent corrosion, wipe off any excess solder after you've made your connection
- Make sure you don't use too much solder on a single point or you could have a cold solder joint
Here's a video for the XL, but it should also apply for 2DS if you use the 2DS NAND pinout at the top of this post when soldering to the motherboard:
Finally, I recommend using Win32DiskImager to dump/write your NAND, as well as this SD adapter to connect it to your computer:
http://www.amazon.com/Anker®-Reader-RS-MMC-Support-Warranty/dp/B006T9B6R2
You might also want to get a 4-pin (or 7-pin) micro JST port (both male and female), so you can easily connect/disconnect your hardmod.
(The 3DS can be picky about how you're connecting the SD adapter to your computer, and a couple of people [myself included] recommend the Anker 8-in-1 reader I linked.)