To get a square corner, sew straight until you get to the point where the corner needs to turn, then stop, make sure the needle is in the fabric, lift the foot and swivel the fabric (using the needle as a axis) until you’re in position to sew the other side of the corner. Lower the foot, and continue sewing.

Round corners are a little trickier- I’m sure there’s an expert way of doing it, but I usually just eyeball it, cross my fingers and sew it slowly and carefully. Notching the fabric to spread the tension in the fabric evenly is essential- I usually do that after I sew, though, because it’s easier to judge where the notches will be needed. And iron, iron, iron- before and after sewing.

Good luck!