All oOnepagers exist within a Collection. You can start a new onepager with a new collection or add them to any existing collection.
You can find the buttons to add new onepager on the Collections page or on the overview page of any collection.
From here you will see a screen that allows you to select or create a new collection. You can also give your onepager a Name and brief Description which will show up inside the header of the onepager.
After you've created the onepager the Builder will open up immediately to start building your onepager.