Copy the .h, .cpp and .resx files into the project directory.
Right click the project name in the solution explorer and use 'Add Existing Item' to add them to the project.
Select the .h file and then in the properties change the file type from "C++ Class" to "C++ Form"

Troubleshooting

missingmanifestresourceexception error

Wth an error message like this:

Make sure "OurLibraries.frmAbout.resources" was correctly embedded or linked into assembly.

The resource file being generated doesn't match what the linker thinks it should be.  Resolutions:

Change the forms namespace to match the main projects namespace – this will typically fix the issue

You edit this setting to change the name of the generated .resx file, although this may cause new issues and make sure you delete the old .resx files as you might find both are being used but of cource only 1 will be being re-generated when you build.

Right-click Form1.resX, and then click Properties.
In the Form1.resX Property Pages dialog box, expand Configuration Properties, and then click General under Managed Resources.
Change the Resource File Name property from $(IntDir)\$(RootNamespace).$(InputName).resources to $(IntDir)\$(RootNamespace).SomethingElse.resources and then click OK.