What you need:
*1 1/2 yards of heavy fabric, like duck or canvas.
*3 yards-ish edge binding. You can use bias tape, twill tape, foldover elastic, or whatever. I used foldover elastic because it's what I had laying around.
*Strapping and buckles (you can reuse the old ones from the stroller) OR plastic snaps.
*Patience and a seam ripper.
*About six inches of 3/8 inch elastic.
*Good polyester thread, like Gutermann or Mettler. Don't cheap out on this one.
2. Rip out all the seams and strapping, making careful note of where they are. Mark them with a marker if you need to. My stroller was so old that the creases were faded and ground in, and I didn't need to mark the seams.
3. Use the old fabric piece as a template for the new one. I would advise that you cut it a little too big, so you don't have to do it twice (like I may or may not have done...)
5. (also optional. If you're not making new straps, skip to step ten), cut out straps in the same general shape as the one you removed. I would have used the old straps, but I couldn't wash the smell out. Plus, I seem to have trouble with pinching chubby little baby thighs in the buckles, so I went the snap route instead. Cut two of the center piece and four of the straps.
6. Sew the straps together, right sides together, leaving a hole for turning. Trim seam allowance, turn, and topstitch.
7. Mark snap placement on the center strap piece. I did mine every 3/4 inch in two rows one inch apart. The reason for putting them so low on this piece is that the top is going to fold over the snaps to minimize a toddler's access to them. Don't want little hands unsnapping now, do we? Install snaps with pliers or a press. Use either all studs or all sockets on this part. I used all studs, because I frequently have more of those than any other part laying around.
8. Add a snap stud to the top corners of the foldover as shown. Add the snap sockets on the bottom so that the top forms a flap that snaps down over the double row of snaps you added earlier.
9. Put snaps on the side straps as shown. I used sockets except for the crossover snaps, since the ones for the belly strap are studs.
11. Find your markings on the old stroller where the straps were and pin the new straps on to the new stroller piece in the same place. Sew the straps on the same way they were on the old stroller. Make sure you backstitch when you do this. Remember- these straps are all that's between a squirmy toddler and a display of crystal barware in a department store! They need to be on there tightly!
12. (Optional- if you're not adding pockets, skip this step) For the pockets- these are up to you. What I did was hem the edges of the larger one. On the smaller one, fold the pocket lengthwise and sew a casing on the folded edge for 3/8" elastic. Then add a large pleat to the side with the raw edge and baste it down. Thread a piece of elastic slightly shorter than the length of the pocket into the casing and sew the edges down. Next, hem the two sides under about 1/4 inch. Sew the small pocket on to the large one as shown here. Pin it to the large one with right sides together and upside down. Stitch the bottom edge of the small pocket. Turn it right side up and stitch the sides, making sure you backstitch at the top. Now pin the large pocket to the back of the stroller piece in your desired location. Sew around the sides and bottom of it twice, backstitching at the top.
13. Now the fun part. Remember all those stitches and seams you marked? Copy them on the new stroller. Don't be surprised or upset if you accidentally sew one of them on backwards or just wrong. Take a deep breath or a coffee break, tear it out, and try again.
14. Mark all the little screw holes from the old stroller on to the new one. Poke holes with the awl through the new stroller in the marked spots, and insert the screws with brackets. Screw the new stroller on to the frame just like the old one.