Hello @kaptnkalle
I only know how it works on steempeak.com - anyway the best frontend ;)
You go to the bottom of a post where you can see the number of upvotes and comments. There are 3 dots next to it. A menu appears with a Cross-Posting option. Important is, that before you can cross post to a community you have to subscribe that community. So in the top area of steempeak.com you find the menu to all the communities.