Right click on the Fediverse (pentagram) icon next to the post or thread you want to respond to, click copy, and paste it in the search bar on your instance. You can then reply.

That's it!

More kludgy stuff:

• You can also subscribe to any community by searching @[community name here]@[instance example.com here].
• Upvoting is handled with stars, but you cannot downvote on Mastodon.