Here is how to download and install the free program Dia, for Windows. I am using the Chrome web browser to download, but the steps will be similar with other browsers too.
Search for the download source (or just click on this link: http://dia-installer.de/)
That’s it. Once you have installed Dia, you can open it by clicking on the start menu (button in the bottom left corner) and it should be under ‘All Programs’