Parson’s style slipcovers are the easiest style to make because they are symmetrical and have no arms.
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The very first step is to make some welt. I have a tutorial on that here. Then cut the strip of fabric that will run along the top edge of the barstool from one side to the other…this is called the boxing piece(pictured below). After cutting the boxing piece, attach the welt to both sides of the boxing strip like the photo above.
The photo above is the boxing strip with the welt attach to both sides. It’s sitting on top of the chair upside down/inside-out because the slipcover will be pinned inside-out. 
Drape your fabric on your chair, smooth it out and tuck it into the seat back just slightly. Make sure to leave enough fabric hanging off the edge to allow for a seam…I usually cut my fabric 2″ beyond the edge of the chair. This may be a little too much waste but it gives me a little wiggle room in case I need it.
The next step is the hardest part of the whole slipcover in my opinion. It’s cutting the notch in the corner so the fabric will lay down flat. You have to cut far enough into the corner so it will lay down, but not too far!
The video will demonstrate this part well.
After the front piece is cut and the corner is notched you’ll cut the back piece. I usually drape my fabric and pin it to the back of the chair with upholstery pins(T-pins), then I cut leaving a 2″ seam allowance around the whole back.
After all your pieces are cut out it’s time to pin it all together. The welt cord makes it easy to feel your way around the piece. I roll the three pieces between my fingers and make sure they’re snug against the corner of the chair and pin. 
Once the whole piece is pinned it’s time to sew. Since this piece is symmetrical and doesn’t have arms there’s no need for a closure because the slipcover just slips right off. Also…notice the front corners I pinned in darts.
Sew all the seams using a welt foot or zipper foot.
The last step is to finish the bottom with either a hem or welt. My chair has velcro on the bottom so I decided to use a welt trim and attach velcro to my slipcover so it’s attached and won’t shift during use. 
That’s all folks…Press the slipcover, install, and enjoy your beautiful new slipcover that can be laundered as much as you want.
