Make sure the settings on your sewing machine are set to sew a straight stitch.

Slide the corner of your fabric into the sewing machine, directly under the hole in the presser foot.

Align the edge of your fabric with the edge of the pressor foot.

This will act as a guide while you sew, and will help you keep you seam straight. In this case, it will leave a quarter-inch seam allowance. You can use a different guide if you would like.

Lower the presser foot using the presser foot lever.