Here is a quick video on how to sew the perfect corner. There is no talking in this tutorial because I was trying to sew a straight line with one hand and hold my phone in the other to video the demonstration. (for the record, this is NOT a professional video, but you'll get the point)
Once you are finished, you may need to clip some of the corners if there is a lot of bulk in the fabric.
When you finish and turn the fabric right side out, you should end up with perfect corners like photo below. Good luck, happy sewing!