Pin the seam you are going to sew.
Sew a scrap of the fabric you are sewing. Refer to your sewing machine manual if the stitches are puckered or the stitching does not appear correct.
Place the end of the seam under the presser foot of your sewing machine, leaving enough fabric behind the needle for a few stitches. Place the raw edge of the seam allowance on your seam guide. How to Set a Sewing Machine Seam Guide for Accurate Seam Allowances
Lower the presser foot.