I haven't been able to do it the correct way... which is find a user at their instance, click the follow button, then type in your username and instance and have it follow. I always get this error:
Error: Invalid URI used for redirect. Warning: Responding with 500 Internal Server Error catcher.
Instead, I have found, from reading the Plume documentation here, how to do it an alternative way that works.
Try typing this into your web browser:
https://hey.dou.bet is your instance and
firstname.lastname@example.org is the username and server name of user at the other instance, Plume or WriteFreely, who(m) you'd like to follow.