If you're using SteemPeak, there's the 3 little dots under his post which will pop up a little action menu. Choose "cross-post in community", another window pops up where you can choose the community to post in and a short description (max 100 char) to go with it.

By cross-posting it it's as if you make a new post itself but actually 90% of the rewards will go to the original author, 5% will go to the community you post it in and 5% will go to you.