The key to creating postcards is picking the right template. If you are going to print your postcards on a specific manufacturer’s card stock, such as Avery 3263 with dimensions of 5.5 x 4.25 inches, then it’s a good idea to find that Avery template or a template with those exact dimensions.
If you are making 2-sided postcards, put things for the front of the postcard on one side of the template, and put things for the back of the postcard on the other side of the template. Then, print them double-sided.
If you are making single-sided postcards, simply print them out.
If you want to use a template other than a manufacturer’s you need to be sure that the postcards will fit on your paper stock. A 5.5 x 8.5 inch template fits two postcards on an 8.5 x 11 inch sheet of paper, and a 4.25 x 5 inch template fits four postcards on an 8.5 x 11 inch sheet.