If you’re on the web UI, this is the crosspost icon https://lemmy.world/pictrs/image/a9918a74-4408-4363-95e3-3cbcc8c6e142.png you’ll find it just below your post title and username etc. All you need to do is select the community you want to cross-post to. You can optionally also add extra text, and tailor the title to make it more relevant.
If you’re on an alternative UI and don’t have this feature, well I think that’s probably why we get so many duplicate posts so it may be worth raising with the developer(s) of your app.