If you have a blog on Blogspot, you can add labels to your blog post. If you do, it’ll say something like the following at the bottom of your posts:
Posted by Rob Conley at 11:39 AM
Labels: ADnD, Advice, ODnD, OSR
You can click these labels. If you click on OSR you’ll get a page that starts with the following:
Showing posts with label OSR. Show all posts
If these are the posts that should go on the Old School RPG Planet, your feed is this:
You can of course use any label you want as long as you are consistent.
See Share content on other sites for more information.