If you prefer creating your designs from scratch without using templates, there's an easy way to get to it. On your Dashboard screen, use the +Create button in the upper right corner and select Create custom design set. You can also use the Search bar to select the same option from the same dropdown menu.
To continue with your design set, you have to select the sizes you want to include. Choose the right sizes from the list of the standard sizes or add your own custom sizes. Click Design set to continue - you can see here how many sizes you selected.
On the next screen, select the first card +Blank set, and all the sizes you initially included in your design set will open in the Editor, where you can customize the design however you want - you can add layers, upload your own images, add elements, presets and animate your designs. You can also add or remove sizes in case you wish to create something else.