First, navigate to the folder that you want to create a feed from. While in the folder, from the Actions menu select RSS Feed. A popup will open to allow you to filter the feed contents by type. Click the button that is appropriate for you (if you are not sure, click All). This will open a new page with the RSS feed. The RSS feed URL is in the address bar of your browser at this point and it will look something like https://app.put.io/rss/all/12345678. You can start using this URL in your RSS client or Synology Download Station.
Here is a demo video of these steps: https://cl.ly/jt1P
One problem that you might run into is that this URL does not contain any authentication information, so will prompt for a username/password when used on a different machine or browser. One simple way to add your account information to the URL is to embed it like this https://username:email@example.com/rss/all/12345678. So if your username is "awesome" and your password is "mysecret", then your URL would be https://awesome:firstname.lastname@example.org/rss/all/12345678.
Another way is to create a new OAuth app from https://app.put.io/settings/account/oauth/apps, get the token from there, and append oauth_token param to RSS URL: https://api.put.io/rss/all/123456?oauth_token=XXXXXXXXX.
Please be careful when using these types of links, though, since they will include your credentials in the open. We advise you to only use these types of links in your own clients.