Make your icons
The exported folder (or hosted app files folder) contains the icon files (e.g. icon_16.png and icon_128.png) which are copied from the Icons folder in the Project Bar. Make sure these are what you want them to be for your listing on the Chrome Web Store.
Upload hosted app
If you're making a hosted app, make sure the exported files are uploaded to your server. You don't need to upload the hosted app files - those are for the Web Store only.
For the game to work correctly offline, you should make sure the server is set up to serve .appcache files with the MIME type text/cache-manifest. To check it's working, visit the URL of the offline.appcache file. Your browser should display a text file with a list of filenames, a bit like this.
Zip your files
You need to create a zip file of your app files. You need to zip different files depending on the kind of app.
Packaged app
Zip all the files in the exported directory. You can do this by selecting them all, right-clicking and selecting Send to - Compressed (ZIP) folder.
Hosted app
You only need to zip the files from the folder specified for Save hosted app files to. There should only be three files in this folder - the two icons and a "manifest.json" file.
Submitting to the Web Store
Go to the Developer Dashboard and click the Add new item button. Click Choose file and choose the zip file you created in the last step. Click Upload. If all goes well, after the upload you should be transferred to a form where you can enter more information for the web store.
The Web Store form
You're not far off having your web app online. Fill out the necessary fields like the detailed description. Make sure you add the required 1280x800 screenshot and 440x280 tile image.
Be sure to Preview changes before you're done to check everything looks OK. Once you're ready, hit Publish changes. You app should now be live!
Using the app
When users visit your app's page (e.g. Space Blaster on the Web Store) they have the option to Install the app. This adds it to the New Tab screen under Apps like this:
Now they're only ever one click away from launching your game. Even better, it'll continue to work even if they go offline! (That is, as long as you left App can work offline checked.)
If your game uses the Fullscreen in browser mode, it will launch in a whole new tab. Otherwise, Chrome launches it in its own new window.
Did you know you can also create shortcuts to apps to make them work more like desktop apps? See How to run Construct 2 games from the desktop with Google Chrome.
Updating your app
If you want to publish a change, make sure you increment the Version in Project Properties, or the Web Store will reject the change. Then you'll have to re-export the project and go through the above steps again, but this time uploading it via the Update file button in the app's edit page.
Payments
You can set up a one-time payment for packaged apps only. However, this won't work for hosted apps - the user could just visit the URL, and there's no good way to secure a javascript app like Construct 2 exports. However, if you have (or know someone with) experience with server-side scripting, it's possible to integrate the Web Store Licensing API. Since this requires server-side code it's not something Construct 2 can enable without the necessary server-side scripts - you must create those yourself.
Server-side scripting is outside the scope of this tutorial. It requires technical knowledge to properly set up, so unfortunately the burden falls on to you to set up a system using the Licensing API. However, it's for the best: it's much more secure that way, and you can make the most of the payment options and better customise it to fit your needs.
Conclusion
The Chrome Web Store is a great place to get your HTML5 game or app listed. Packaged apps are probably most convenient for most games, since you don't need any hosting - the Web Store hosts the files for you.
Remember you'll still need to publicise your game to let everyone know it's there! Don't rely on people accidentally finding it on the Web Store - it's crowded with a lot of competing apps, so you'll still need to let the world know about it.