.NET Micro Framework is open source system provided by Microsoft.
You can download the full source codes for free from Microsoft, search for “Micro Framework Porting Kit”
GHI also provides a porting book to help you out. The book also shows you all steps you need to DIY NETMF board
See the porting book here
http://www.tinyclr.com/downloads/Beginners%20Guide%20to%20Porting%20NETMF.pdf
Note that GHI’s NETMF devices have a lot of features added on top of what is standard in NETMF code, like WiFi and database. This is the link to GHI libraries added to NETMF
http://ghielectronics.com/downloads/NETMF/Library%20Documentation/Index.html
The FAQ also covers a lot of details on NETMF FAQ – GHI Electronics