If your highchair has a cloth pad, we're betting it's seen better days. Even if it's washable, it's just not realistic to get it stain-treated and in the washer after every messy meal. Bethany's son Jakson "thrashed" his highchair cover so she sewed a new one from oilcloth that she can wipe off. And, she's sharing instructions to help other parents in the same boat.
You may be able to purchase a replacement pad for your chair (remember this Good Question?), but making your own is likely cheaper and not very difficult. It's also a great way to give new life to a used or thrifted highchair either for your own home or to keep at a grandparent's or caregiver's home.
Bethany used oilcloth, bias trim tape, denim needles and thin foam. Her chair is a Graco and she recommends using your original cushion or pad as a template. See her instructions, tips and more photos on her blog Vermillion Rules.