Update March 2023: If you want to cross-post from something supported by brid.gy, I recommend using that service since it’s a lot simpler to set up. But if you want to pull from an RSS/Atom feed, or anything else that brid.gy doesn’t support, or customize features like post visibility, this will let...
Webhooks can no longer be used with IFTTT with a free account.
If you want to publish an RSS feed on Mastodon, you can take a look at https://gitlab.com/chaica/feed2toot. However, you have to host the solution yourself.