The feature installation of the Graphical Process Designer is as simple as it can be. Assume you have installed your Eclipse platform by extracting the Eclipse download in a folder ${my-eclipse-folder}. If all went well you will observe a folder ${my-eclipse-folder}/eclipse which has in turn two subfolders called 'plugins' and 'features'.
To install the GPD feature, you simply extract the downloaded zip file (containing this readme.html file) and choose the folder ${my-eclipse-folder} as the target for this extraction. This extraction will add a feature and a number of plugins in the above cited folders, overwriting any folders or files with the same name that might be there already.
But be careful : it is possible that the overwriting of files mentioned above raises compatibility problems afterwards, so do this only if you know what you are doing. The recommended solution is to do the extraction in a fresh Eclipse installation, as this will not cause any problems.