How to Set Up a Folded Card

There are two ways you can go about creating a bi-folding card like this--one is to use just one artboard, and the other is to use a 2-artboard layout.

Single Artboard Layout

If you have an element (like a single flower image, for example) that you want to have extending from the front onto the back of the card (crossing over the fold line), then you may want to use just one artboard for the design (see image below). You will have two Pages in the design--one page for the front/back of the card, and the second page is for the inside of the card:

If choose this option, then you will also want to add a line using the Shapes tools (see above) so that customers can see where the card will fold. You can set this line to Hide in Download in the right-click menu for the line shape so the line doesn't show up when the customer downloads the design.

Two (2) Artboard Layout

If you don't have any elements that you want to overlap onto another side of the card, then the second way to set this up is to use 2-artboards when creating your design.
For a 5x7-inch card that will fold into a 3.5x5-inch card, you want to click on the Custom option in the Choose an Artboard Size area, and size the artboard at a Width of 5-inches and Height of 3.5-inches. This will give you the same setup as above, only now you have a different artboard for each section of the card:

You don't have to add a line to this version because customers can see the outline of the different artboards to know where the fold in the card is.

With this option each artboard will contain its own Layers that will be shown when the customer clicks on that artboard.