I didn't write a detailed tutorial for this one because it's hard to give accurate measurements since all highchairs are different, but here are the basic steps in case you want to make your own...
Oil cloth (because it's wipeable) --you can find this all over the internet
Quilt binding (bias tape) --I used the extra wide because I'd never done this before, but I think I'd use something a little skinnier if I were to do the project again
Denim needles --DO NOT try and use regular "all purpose" sewing needles-they will break!! Take it from me, I made that mistake
Foam --you can decide on the thickness you want, but I used the skinniest foam Joanns' had (I believe it was 1/2" thick?)
**Tips and tricks from the experts--you may want to read a little bit about oilcloth if this is your first time sewing with it. I found some helpful advice on Sew Mama Sew!
Lay your original high chair cover on butcher paper and trace in two parts. Trace the seat as a separate piece and the back as a separate piece. You don't need to include a seam allowance when you trace.
Lay the pattern pieces down on your oilcloth and cut two of each pattern piece (two backs and two seats)
Cut one piece of foam for the back, making it about 1/2 inch smaller all the way around (this way you are not forcing your sewing machine to stitch through the binding, two layers of oilcloth AND the foam!) and one piece of foam for the seat (same thing, smaller all the way around). Use your original seat cover as a guide for the foam. For example, on mine the flaps where the child's legs stick out of the high chair didn't have any foam in them, so I cut that part off of the foam piece
Lay out your stacked layers on the floor---oilcloth, foam, oilcloth. Cut off any excess you see (or where they don't match up quite right)
Pin the bias tape to your layers. I did a section at a time, leaving the top of the seat and the bottom of the back piece raw. You will be sewing these two parts together later.
Use a zig zag stitch (very forgiving in case straight lines are not your forte) to sew close to the inside edges of the bias tape.
Sew the bias tape all around the back and seat pieces (leaving the aforementioned edges alone-hopefully this part makes sense as I don't have a picture of it).
Once the bias tape is sewn on, you only have one seam left!! Put right sides together (the right sides are the sides that you want to show when the cover is in the high chair). Match up the seat and back piece along the raw edges and use a straight stitch to make a seam.
Voila! Completely unique and custom high chair cover!!