After some grinding I figured it out and here goes:
In C# Visual Studio, create a new class library, name it, add your code, add references as required, then build it. If it builds fine, close the solution and save the project in a chosen folder. In explorer, find the directory where the solution was saved, find the bin/release subfolder, copy the .dll file to RE plugin folder.
A good start is the sample indicators find on RE website.