Step 1. Create project
Open MonoDevelop and create a new Library project.
Enter the project name and choose a location path.
Step 3. Rename
MyClass.cs will automatically be created. You can rename it, just set cursor to class name (line 5) and press cmd + R
Step 4. Write code
Write your code and try to build (cmd+B). If you use UnityEditor, UnityEngine and etc. standard libraries, you will receive references error.
Step 5. Add references
Right click on References folder and select Edit references...
In opened window select .Net Assembly and press Browse...
You can find unity libraries in Unity application folder.
Step 6. Options
It allows use your library in unity
Right click on Project folder and select Options
In opened window select Build -> General and change target framework to Mono / .Net 2.0
Step 7. Build
.. and find DLL file in project's folder
Step 8. Use library
Import your namespace and use your library. Good luck!