Under your collection page there will be a green (create a new collection) button. After you click that button you can name and then create your new collection (blue create button). You will now see on your collection page that you have multiple collections to choose from. Whenever you want to work on a collection, be sure to click on that collections button and you can add/update cards to that specific collection. If you look at the navigation heirarchy when you navigate to a collection you can see what collection you are working on in the format Main / Profile / Collections / (here will be the name of whatever collection you are woking on).
A word of caution when working with multiple collections, be sure you are actually working on the collection you intend to be working on. It's easy with multiple collections and bouncing back and forth between them to be working on the wrong collection. When you navigate to a specific set, name or team to work on there will be a dropdown menu under collection summary giving you access to all your different collections so you can be sure you work on the correct one.
My Autograph Collection Website Emphasis is on the 1969 Cubs. This is still a work in progress as is my collection here at TCDB.
Value what you collect, don't collect for value.