The process is fairly simple, but takes time. When building your project, you can see options for what platform you want to build to. You need to choose WebGL, switch platform (this takes 5-10 minutes) and the build your game (also a bit of time). When uploading it to itch.io, you need to select HTML at the "Kind of project" section, upload a ZIP of your file, and below that you will need to select "this will be played in your browser". You can add more files that will be registered as downloadable too.