This is the chair before. A plain and boring gray and black chair.
Next, measure the seat cushion. If yours is like mine, think of it as a square and forget about the round corners for a minute.
Insert a button hole to feed your cord through, to gather the fabric and tie it over the seat cushion.
Half way done!
Mine was too big and loose. I used a pen to mark the fabric and made some rough marks to know where to sew to tighten up the slipcover. (Arrows are pointing to my pen markings).
Sew along your markings. I think I repeated this step twice to get just the right fit. (Pictured below)