Your on the right track, you need to have them in the same project, and have the game reference the project (not .dll) directly. To do this, go to your main game, select Add Reference
.
When the dialog pops up, find Solution
, and under it Projects
. You should see your library in there, tick the checkbox and it will be referenced.
As far as the solution layout goes, and how to use the library in your game, I created a quick illustration: