When you create a new collection, you see an option to add stamps to it. (Please see add a new collection section for detailed instructions.)
Click on Add a Stamp button to add a stamp to your collection.
The right sidebar appears with Add Stamps section. If you have added or published stamps in your account, the stamps appear as below.
Click on Add button available in each stamp to add it to this collection.
As you click on Add button, the Remove link appears. Click on Remove link to remove this stamp from the collection.