I saw on a blog a while ago (that I don't even remember now, sorry!) where a mom had actually upholstered her bar stools with oilcloth so she could clean them easily.
When Lydia switched from a high chair to a booster, my mom mentioned I should put something under the booster to protect the chair it was sitting on. I remembered the blog with the oilcloth chairs and a light bulb went off!
I measured my chair and made a pattern of it on wide freezer paper. I also measured how much oilcloth I would need to wrap around the seat of the chair and attach to the bottom of it (sorry I didn't take any pictures of this part!)
Then I cut out the pattern from the freezer paper and tested it on my chair to make sure it fit and covered all the parts I wanted it to. Then I laid the pattern on my oilcloth and traced. In this case, it doesn't really matter if you trace on the right or the wrong side of the oilcloth, but I would recommend tracing on the wrong side so you don't see your trace marks.
I then cut the pattern from the oilcloth. On the bottom of the chair where I wanted the oilcloth to wrap and attach, I put one-ish inch pieces of sticky Velcro. Leave both pieces of Velcro sticking to each other so that when you wrap your oilcloth under the chair it will automatically stick to the exposed adhesive. Like so:
And that's it!
And whenever I want to remove the oilcloth, I just separate the Velcro, like this, and nobody would ever know that that chair is the one that gets doused with milk, yogurt, fruit, and mac and cheese!
I ordered my oilcloth from Oilcloth Addict on Etsy, which has a great selection.