These are classic chairs, characterized by their cool perforated vinyl seats. Unfortunately, you've gotta know where to look to find new upholstery and old stiff foam is tough to remove. I'll show you how.
Step 1: Set Up
Scissors, a sharpie, a screwdriver, pliers, a wrench, a bunch of rubber tipped clamps, a metal spackling tool, weights (dressmaker's will work),
a 2-3" paint brush, rags, some newspaper or a drop cloth (this gets messy), Barge cement, and Barge thinner and a ventilator is always a good idea when working with strong adhesives.
Purchase 24" x 24" x 1" Foam and head to your local Auto Upholstery supplier for a yard of perforated vinyl (headliner material).