At RSS Mixer enter the URL of the site you want to grab a feed from (if it’s only got one feed, you probably don’t need to enter the RSS feed URL, just the site):

Add more feeds by clicking on the black + button:

Once you’re done, click on the Mix it button.

Click on the orange feed button to grab the mixed feed:

If you sign up (free and painless) you can do other things, such as go back and edit the mix, add new ones, etc.

If you download the Firefox extension you can add feeds from the page you’re browsing to an existing mix or a new one.

Click on the button and you’ll be taken to a page, giving you the option of starting a new mix, or adding to an existing one:

Alternatively, look through the lists of feeds other people have added, select one you like and click on the Add to a mix button: