Once you have the code typed up and your ready to test it the first thing you need to do is Build the Solution.
At the top of your Visual Studios Application you should have a section Labeled BUILD
Click on BUILD
Click on Build Solution
Look for the Output window usually at the bottom
Verify no errors
Navigate to the location the .dll was saved, if you dont know the it should be listed in the output window.
In my example this is set up in my lab and i have Visual Studios installed on the same server as my Synchronization Service. If the file was not saved to the extensions folder of the Synchronization Service you would want to move the file to that destination.
Once the file has been moved to that directory open the synchronization service.