Comment by dredmorbius

The mechanism is not friendly or intuitive but:

1. On your own instance ...

2. Paste "@paulg@mas.to" into the search dialogue and click the magnifying glass ...

3. Paul's profile will pop up in the results under "People".

4. Either click on the person icon to follow directly, or ...

5. Click on the avatar / profile name/description to view the profile page itself.

If you do click the "Follow" icon from mas.to (and don't already have an account there), you'll be prompted to do what I've described above.

Keep in mind that the Fediverse is, well, Federated. Someone else's home instance is where their bits and their configuration live. Your instance is where your configuration lives. You subscribe from your instance for that reason.

Some instances block others, in which case the profile won't appear, though odds are low that mas.to is among those yours has blocked.

(I've been on Mastodon since 2016, yes, this was confusing at first. I've since sorted it out.)