Cross-posting itself is simple.

You just click "Cross post in", indicate the destination community and a note that goes with the cross post (much like a retweet with comment on Twitter, if you are familiar).

There are a few things to be mindful about, that's what makes it less simple.

• currently only Steempeak displays them properly, on other interfaces they will look like a post with the original title and the content of "This is a cross post of [link] by [cross-poster]. [your note]".
• the rules of the community about cross-posting
• the original authors will only see the engagement on the cross-posted posts if they check their mentions. There won't be notifications about new comments, because they go to the cross-poster.