Just fill in the fields and click on the Generate button to download your marker, or click on the Generate All button to download them all in a row.
If an inserted image isn't displayed on your downloaded icon, it's because the server doesn't allow to load that image outside of their domain. In this case you can use another host for that image, e.g. Imgur. Check out this video for an example:
At the end of the page you can export your configuration by copying the configuration text and save it somewhere. If you want, you can import it later by pasting it into the box also at the bottom of the page.
Tested in Chrome, on desktop. Disclaimer: as the whole generation process runs in your browser, the result can be highly different from time to time. :) If the generated icon isn't perfect, it's because of the third party generator library. Just try again.