Using feedsin.space is pretty straightforward. First, you need to authenticate with the website by sending a message from your mastodon account to @admin@feedsin.space with the word "help". You'll get a response with a link you can click on to authenticate with the website. Then, you can create an account on feedsin.space by specifying a username for the account and the RSS feed you want to follow. Assuming everything looks good, there will be an account created at @username@feedsin.space, which you can follow from your Mastodon account,

Steps to use Feeds in Space

Send the word "help" to the @admin@feedsin.space account. In the screenshot, I'm sending a DM / private mention

The bot will respond with a login link, also via DM

I'm now logged in and can add feeds

If I hit logout, this is the screen I see. Basically, you follow the process of sending a help message every time to authenticate. My recommendation is that you do this as a DM to the admin account.

Note: currently, I haven't gotten any RSS feeds to validate correctly, so that may not be working