Use your left hand to guide the fabric by placing your little finger more toward the back of the machine and your thumb to the front. Let your other fingers lay down comfortably somewhere in between. Only light pressure with all of your fingers letting your hand move to back of machine as it stitches. For now stop and reposition your hand for the next segment. Practice on some scraps first. Don't be hard on yourself. Sewing a straight seam is more difficult than it sounds. Your right hand also helps guide, but the left does a lot of the work. Kudos to you for asking for help. I read this here on the board or somewhere.