Open Visualizer
After you have a visualizer installed/placed in the right folder. You can access the visualizer by starting DF Storyteller using the "start" subcommand. After the service has stated go to: http://127.0.0.1:20350/paintings/ ( Note the URL might be slightly different depending on your configurations. ) Here you can find you personal collection of paintings/visualizers. You can always at more visualizers later on. It will work automatically with all (already) imported worlds.
Direct opening
If your don't want to use your private gallery you can always go to the following URL depending on the visualizer.
http://127.0.0.1:20350/paintings/[foldername]/[foldername] should be replaced with the exact name of the folder inside the "serve-paintings" folder.
Non static visualizers
The case above assumes that the visualizer you are using is build using default web technologies like HTML, CSS and JS or even Web Assembly. But there could be visualizer that are written more like desktop applications. In these cases you have to follow the instructions on the website or post you downloaded it from.
Note for developers: We might add the ability to execute native application visualizers from the gallery in the future. Please give us your suggestions here. For more info for painters: Become a painter.