Creating a new post in Universe is extremely simple. Just click on the new post field and start typing. When you start typing, all the rich text editor controls will appear.
You can also tag channel members to the post easily by typing @ and then selecting the user you want to tag from the list.
Adding attachments to the post is also easy. Just click where you want to attach the file from, for example, Google Drive or your computer. Then just select the file you want to add and click "Select".
When you share the new post Universe will also check any Google Drive file permissions and notify if the file needs to be shared with the channel participants.