In your project there is a file called /Content/Content.mgcb
This is like the Content.contentproj
About the format of this file see 'Response File' in this link:
You can edit it using Pipeline.exe which comes with the MonoGameSetup.exe SDK.
I think it has an option to import .contentproj but never used it.
The idea is to have your assets inside /MyProject/Content/
but I assume you already have your assets in /MyProjectContent/
If you want to keep your assets separate and not duplicate them you could add them as links in .mgcb,
personally I've never done that.
What I did instead was to move the .mgcb file in the content folder.
/MyProject/Content/Content.MGCB -> /MyProjectContent/Content.Android.MGCB
and edit my project MyProject.Android.csproj to add the .mgcb as a link like this:
You can add custom importers/Processors in the .mgcb using
You can use the Pipeline.EXE UI tool to do that.
You'd probably need to recompile those for MG, In theory they would work as they are but anyways..
Create a new C# Library project and your .cs files and a reference to the portable version of MonoGame.Framework.Content.Pipeline.dll and if needed to the portable MonoGame.Framework.dll
In most of my projects I am still using the initial portable library 184.108.40.206-Beta from nuget, however there is now an official latest release if you want to try those.
Make sure to change the 'Copy Local' to false (
<Private>False</Private> in the .csproj).
If your library is using 3rd party libraries or reference any other of your libraries the build will fail, it's a bug in MG 3.7.1. Use the latest Develop build (also try to keep a working/stable MonoGameSetup.exe around because develop builds ofter comes with new bugs from time to time).