There are a couple of different ways you could proceed from here.
Pictured Above: Stitch around the side, bottom and other side using a 1/2" seam allowance.
An Alternate Method (Not Pictured): Leave a bit unstitched at both upper edges, but otherwise stitch all the way around the side, bottom and other side as pictured above.
In a later step, you will be turning the upper edges down, and if you leave a bit unstitched it will help make that part of the process easier. The downside is that it's hard to know precisely how much to leave unstitched. In my sample project, the unstitched length should've been about 3/8 of an inch, but your project might require a slightly different measurement.