Making a flat in Illustrator is a quick and easy process. To create a flat, open the Illustrator document you wish to create the flat in. Next, select the Rectangle Tool (M) and draw a rectangle on the artboard. Next, select the Align Rectangle to Path Tool (V) and click and drag the bottom-left corner of the rectangle up and to the right, as shown in the screenshot below.
This will create a flat. To finish off the flat, use the Type Tool (T) to type in your desired text, and then save the file.