Am I supposed to include the entire post in the description field? Or just a summary?

Lots of folks just include everything up until a reasonable character limit like 1000 characters. I think most feed readers will cut long descriptions short.

It is best practice to include an intentional separate summary. I write a sentence into my metadata for this. I try to answer "how would I describe this article if I sent it to a colleague?"

How do I handle the markup and formatting of the post?

The content should be HTML, and should generally only contain what would go in the main "content" tag in an accessible website. (Be sure not to include site navigation, page footer, etc)

Am I supposed to delete items from the feed or do I just keep appending items to the bottom of it?

I just keep adding crap until the end over time.

Technically, I clean it up, occasionally, because my RSS feeds and best of my blog links are built from the same metadata.

Its it good practice to include a thumbnail image or anything with the item in the feed?

Yes, it's a good practice. People like images.

How is that possible

A valid HTML image tag anywhere in the RSS will typically be rendered in a reader.

Some RSS specifications include a place to specify a preferred image. Readers vary in how they use this image, but will typically feature it prominently. (Because people like images.)

Traditionally, this image should be particularly relevant to the content, and should also actually be included in the page content. (I feel like this rarely happens, though!)

Most feed readers will simply choose the first image they find on the page, if nothing is specified in the specific image RSS field. This often gets weird, especially on websites with prominent ads.

How do I distribute the feed beyond adding a button on my site?

https://mastofeed.org/ can automatically post your RSS feed links to your Mastodon account, if you're into that sort of thing.

That said, I don't actually want more blog readers, I just think the RSS spec is interesting. So take my advice on gaining readership with a huge dose of salt.