This is a part of the instructions that needs more detail I am afraid. Have you found the part of the instructions for the Sangaboard compatible motor controller using an Arduino Nano? This includes a wiring diagram and a link to the repository with the Sangaboard firmware for Arduino. The repository instructions contain steps that you do not need if you have a Nano. Find the download button at the top of the page
, next to the blue Clone button. Click on it and click the zip button under download this directory.
Then unzip the file and open sangaboard.ino in your Arduino IDE. If you have not used Arduino before see the Arduino web site on how to get and use the IDE and load programs to your Nano.
There is a useful picture on a recent thread of how the Nano and motor controller boards fit on the nano_converter_plate.stl, First build - High Resolution OFM.
I hope this helps. There is are plans to simplify this process but we are not quite there yet.