Step 20: Appendix B: Makeshift MicroSD Card Holder
Take the MicroSD-to-SD adapter, which should have 9 pads on it like a SD card should. Get some standard 0.1" pitch male header pins, and cut off a 9 pin long section. It should fit just perfectly against the pads. Solder this header to the pads.
Tip: first do one pad on one end, and keep the solder liquid while ensuring the header is aligned correctly, and then allow the solder to cool and solidify. This ensures that the header is straight. Then solder the other pads. Be careful as the plastic MicroSD-to-SD adapter can melt.
If you do not want to use my makeshift method, then purchase a real SD card holder or MicroSD card holder, the pinout should be provided by the manufacture. Many companies carry several breakout boards for these holders.