First create a Multimedia Resource Category, give it a good title since this will be the name of your Podcast. In the category details you can summarize the content, rate the podcast, provide an image, etc. Then you should upload the multimedia files to your website and associate them with this category. Whenever you want to add a new episode to your Podcast, you just upload a file and associate it with that category.
When you have some files in your Podcast category, you can view the RSS feed, to make sure it's showing up correctly. To do this, you need to know the ID of the category. The easiest way to find this out is to go to your Multimedia Category Admin page. There you will see a column that says "Category ID". This is the ID, but as a shortcut you can just click on the ID and it will open up a link to the Podcast feed.
The URL will look something like this:
You can copy the URL and then test the feed in iTunes by going to the menu: Advanced > Subscribe to Podcast... then enter the URL. If the feed is valid, iTunes will display the feed and you'll see your episodes. After you have confirmed that the feed is valid, you can submit the URL to iTunes. Their site has detailed instructions:
Once your site has been submitted and approved, you will be given a Podcast ID from apple. You can go back into your Multimedia Category and enter this Podcast ID under your Extras tab. Then go to your Advanced tab and make sure that you have the RSS Feed selected as one of your display elements for the list page (If you're using default display elements, you dont have to worry about adding the RSS Feed). Once you've done this an iTunes Podcast icon will show up on your website for this multimedia list, so people can subscribe directly from your site.