Summarize:

Flip the cover right side out so that the seams are hidden, and leave the strip inside out so that the seams are visible. Locate the seams on the cover and the strip. Line up 1 seam of the strip with 1 seam of the cover. Position the strip along the outside of the bottom of the cover so that their edges are aligned. Place pins through the strip and cover, starting at the seam. Insert the pins perpendicular to the edge of the fabric to make it easier to remove them as you sew. Keep in mind that some types of stretch fabric can be slippery! It might take a few tries to line up the edges just right. Position the edge of the cover and strip under the presser foot and lower it. Then, apply gentle pressure to the pedal to sew forward. Sew all the way around the edges of the cover and strip. Make sure to remove the pins as you sew. Do not sew over them! Cut the excess thread when you reach the end. After you have finished sewing the 2 pieces together, flip the strip over so that the seams are hidden again. Then slip the car seat cover over the car seat so that the edges stretch over the outside of the seat and the opening is positioned over the car seat handle. Always put the car seat cover on after baby is in the car seat, and remove it before you take baby out of the car seat.
Pin the strip and cover together starting at the matching side seams. Sew a zigzag stitch along the edges of the cover and strip. Turn the pieces right side out and slip the cover onto your car seat.