You can do this by making a small nip in the seam allowance or using a fabric marking pencil. First step is to thread your sewing machine and have a new, sharp needle in place. Needles do get dull and are the most common cause of machine malfunction and poorly formed stitches. If you do not have the manual to your sewing machine please refer to Locating A Sewing Machine Manual or ask for help on the forum . There are many teachers, with all kinds of machines, out there waiting to give you a hand.
Most sewing machines have many different sewing machine feet.
If you have not used a foot before it is always a good idea to practice on scrapes of fabric before starting on your actual project. To sew the hems of the pillow cover you will want to make your stitches and even distance from the end edge and as close to the unattached fold as possible. Making the stitches close to the edge will help the hem from turning. You may want to refer to your sewing machine manual to learn more about the various feet and needle position capabilities of your sewing machine. If you are not familiar with operating and controlling a sewing machine, please take the time to practice using the ideas described in Mastering A Sewing Machine .