You will need fabric, iron on vinyl (optional), boning, and velcro. I cut the fabric as follows:
(2) 11.5 in x 14 in (one piece lining and one piece main fabric)
(2) 13.5 in x 14 in (one piece lining and one piece main fabric)
(2) strips 2.5 in x 14 in for straps- the length they are will depend on your car- I measured my specific seat before I picked a length for the straps.
Then cut the vinyl the same size as the lining pieces and iron it on. (I wish the blocks on the back of the vinyl were 1" squares... that would make things less complicated)
Then find the middle of the larger pieces of fabric and mark 2 lines to each side of it- one inch over and 2 inches over each direction. This will help you make an even pleat.
Fold the fabric so that the lines meet (you will be essentially removing 2 inches of fabric by doing this, which will make the front and back line up)
Pin the pleat in place
Pin the front to the back with the pleat at the bottom. It is easiest to do this by pining both sides and then the bottom. Remember the top will be larger to help with the opening. Do this to the lining and main fabric. Sew the sides and bottom of the lining and main fabric. Leave a 3 or 4 in opening down the side of the lining fabric to turn inside out later. Set aside.
Now fold the strips of fabric right sides together and sew down the edge, turn inside out (which I really don't like doing) and iron flat.
I forgot a picture, but turn one end of each strip in and finish the edge.
Now place the main fabric inside the lining so that right sides are together and pin the top. I start at the seams so that they line up and then pin around the rest.
Now place the straps in between the 2 pieces, equal distances from the seam and pin. Sew around the top and turn right side out. Then topstitch the edge around the top. You can see it in the next picture, but I start getting lazy at the end of a project and don't do great taking step by step photos... sorry.
Now there are several steps missing, but I sewed about 1/2 in down each side from the top down the seam in order to make the sides for a little pocket for the boning which will hold the bag open. I should have taken a picture because it's hard to explain. I then cut the boning the same size as the front of the bag and placed it inside the opening in the lining. Pin it to the top of the bag and sew a stitch under it to hold it in place. You can now sew the lining closed also. That brings us to this picture:
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