Normally I’d start by going to https://www.openstreetmap.org/ and selecting “help” there. That’ll take you to https://www.openstreetmap.org/help - the “welcome” link will show you a short introduction (including to make sure that the information that you want to add isn’t from a copyright source). Down the bottom of that page there is a “start mapping” button.
In your case however searching OSM for “Davaar Island” finds https://www.openstreetmap.org/way/104809516/history , so it’s already been added, but it looks like someone’s forgotten to exclude it from the Firth of Clyde. I’ve mentioned that at https://www.openstreetmap.org/changeset/71238687 .
Best Regards,
Andy