2 yards of fabric
Thread to match
Step 1: Trim your piece of fabric. I tore 1 inch off of the edge with the white strip with the little colored dots (I don't know what it's technical name is). I also tore off a three inch strip off the length (so instead of it being 36 inches it is now 33 inches long). So my final rectangle was 40.5"X 33". Repeat for your second piece of fabric.
Step 2: Fold your fabric over hamburger style (so not the long tall skinny way, but the short way)
Step 3: Place one piece of fabric on top of the other making sure all the edges line up.
Step 4: I made mine with rounded corners. To make a pattern for rounding your corners you can use a forth of of circle, place a bowl or a plate on your corner, or eye ball it I guess. In any case cut off both of your top corners only. Don't cut the corners on the folded edge.
Step 5: With right sides together pin and sew all around the outside edge of the fabric using a 3/8 inch seam, leaving an opening on one edge for turning. Backstitch at the start and finish of the opening. Trim with pinking shears around your rounded corners before turning to reduce bulk. Turn right side out. Press flat with an iron being sure to press out the seams completely (meaning that there is nothing folded inward)
Step 6: Topstitch all around the outside of the fabric using a 1/8 inch seam.
Step 7: Now to make the attachments. Using the 3" strips you tore off at the beginning, iron them flat and fold them in half. Cut along the fold to get 2 pieces.
Step 8: Fold each of those pieces in half and cut along the fold again. You should now have 4 pieces of each fabric.
Step 9: Place one piece of each fabric right sides together and sew along the sides with a 1/4" seam. Turn them right side out and press flat.
Step 10: Fold down a little over an inch of fabric towards the inside at each end of the strip. You should end up with a strip that is 8 1/4 inches long. Top stitch around all sides of this strip with an 1/8" seam.
Step 11: Placing velcro. Cut two pieces of velcro 1 1/4 " long. Separate. Place the soft pieces together at the top of the strip like this (see picture) and sew all around the outside as well as down the middle (in the middle between the two pieces catching a bit of each piece).
Place the rough end of the velcro at the bottom on the opposite side like this (the first one was 1 inch from the top of the strip and the second one was 2 1/2 inches from the top). Sew around each one. Velcro placement can vary as you wish. I like this placement especially if you are planning to give this to someone else. Every carseat is different and this allows the person to have some flexibility with the tightness or looseness of the attachments. I like mine tight so the cover doesn't slip all over the place. To make the second set repeat the exact same process only on the opposite sides of the fabric strips.
Fold your attachment strips in half also and press with the iron. Match the center fold of the strip to the center fold of the large piece. I placed my attachment pieces 8 1/2 inches from each edge. Sew along the fold, backstitching at the start and stop. Then flip the fabric over and place the other set of strips on the other side the same way and sew through them making sure that the ones you just sewed previously don't get folded over or tucked under.