It may sound silly or obvious, but: Build it using the Windows CE SDK (download link).
Step 1: After installing the SDK, create a new Windows CE Project.
Step 2: Take the blank Form1.cs form in the new CE Project and rename it to the Form you use in your old Windows Mobile 6 Project.
Almost done.
Step 3: Now, close Visual Studio, browse to the Windows CE Project Folder, and delete the that form you just renamed.
Step 4: Browse to your Windows Mobile 6 Project Folder. Copy everything except for the bin
and obj
folders and the .proj and .sln files to the Windows CE Project Folder.
Step 5: Open Visual Studio and the Windows CE Project.
Step 6: Get crazy and hit Run [F5].
This will most likely produce a number of errors. These errors are because you have called a function that is specific to Windows Mobile or it could be because the Windows CE version of the same function requires different parameters.
Step 7: Some things may not be easily ported to Windows CE. For those items, you can start new threads like "How do I do FunctionWM() using Windows CE?" or such.