.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
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
The FAQ also covers a lot of details on NETMF http://www.tinyclr.com/faq/