There are two ways to add a post to a collection. First, during the New Piece flow and second, from the Published page.
Option 1: On a New Piece
Once you begin to create a New Piece you can click the 3 dots to the right of Publish and select "Add to Collection".
A drop down will appear with your available Collections. Click on the one you'd like to add your piece to and it's done!
Option 2: An already published post
Go to your Published work.
Click the 3 dots on the bottom of the post you want to add.
Add to Collection will be an option, simply select it and it's done!
Note: posts can be added to multiple collections.