Having made a couple dozen slip covers over the years, I thought I'd share my thoughts my favorite technique you can employ to make your own. In this example I used my newest fabric collection for Art Gallery Fabrics, Fairview Avenue. The piece I used is
Innocent Blooms -Rustic. 

Bari J. Fabrics can be found worldwide in Quilt Shops. (If your shop carries this collection, please let us know to be added to the list.)

You can find my favorite slipcover making supplies here.

(Note: I make a small commission from each product sold from the above link.)

This is a straight forward piece to cover because it's a simple, straight lined parson's chair. It's a great piece for beginners with no curves. 

To start, I'm using quilt weight fabric here. So I used fused on woven cotton fusible interfacing to the wrong side of the fabric for the back and for the seat. I did not use interfacing on the skirt. 

You will need the following to make a Parson's Chair slipcover with a box pleated skirt:

• Simple Parson's chair with no major curves
• Bari J. fabric such as Fairview Avenue (about 3-4 yards per chair)
• Woven cotton fusible interfacing
• Matching Thread
• Straight pins
• Tailor's Chalk or Frixion Pen (always test fabric with frixion first)
• Measuring Tape
• Iron and Ironing Board
• Scissors and rotary cutter
• Quilting ruler
• Sewing machine

Here's my basic method for creating this slipcover:

Drape and fit the seat section

1. Cut a large rectangle that will cover the seat with at least 2" extra at all sides and where the seat meets the back of the chair.
   
2. Lay the fabric wrong side out over the seat only. Smooth firmly across the seat top.
3. At the front corners, pinch and pin vertical seams down the chair legs to create a tailored line.
4. Mark the seams with tailors chalk or a frixion pen.
5. Mark along the bottom edges as well as the back edge of the seat where it meets the back of the chair using the chair itself as a guide.
6. Remove the seat section, mark a 1/2" seam allowance and trim.

Sew the seat section

1. Sew the front boxed corners. Trim the excess fabric and clip the corner to but not through the seam. 
2. Test fit on the chair seat and adjust seams as needed for a snug tailored fit.
3. Set the finished seat unit aside.

Drape and fit the back section

1. Cut a large rectangle to cover the entire back from front to back with extra for your seam allowance.
2. Drape wrong side out over the chair back.
3. Smooth the fabric over the front and back of the chair back.
4. At the two back corners pinch the excess fabric and pin vertically. Mark the seam lines nice and tight.
5. Continue smoothing and pinning until the panel fits like a tailored sleeve.
6. Mark with tailors chalk or a frixion pen.
7. Remove, add seam allowance and trim.

Sew the back section

1. Sew along your marked lines.
2. Turn right side out and test fit on the chair back. Take in or let out seams as needed.

Join the seat section to the back section

1. Place the seat section on the chair seat, right side side out.
2. Smooth so this is nice and tight and then mark where the seat will meet the back of the chair.
3. Remove and add a 1/2" seam allowance.
4. Repeat this step for the back of the chair.
5. Place the two pieces right sides together and sew your seam allowance.

You should now have one unified piece, perfectly fitted.

Create the box pleated skirt.

Measure

1. Measure the perimeter of the slipcover at the seam where the skirt will attach.
2. Measure the length to the bottom of the skirt and add 8 1/2" for a hem and seam allowance.

Calculate pleats

When you are planning a box pleated skirt, you need enough fabric to cover the perimeter of the chair, plus extra fabric for each pleat. Here's a simple example so you can see exactly how to do the math.

Example:

Let's imagine your chair measures 20" on all sides. 

20 x 4 = 80" of chair perimeter

Now let's add the pleats.

Each of my pleats uses:

• 2" of pleat face (the part you can see)
• 2" folded behind the left
• 2" folded behind on the right

So the total fabric consumed by one pleat is 2x3 = 6"

You have 20" to cover on each side of the chair.

• Each pleat is 2".
• 20/2=10 pleats total each side.
• 10x4 = 40 pleats.
• Each pleat takes 6" of fabric to make.
• 6" x 40 pleats = 240" of fabric needed
• add 1" for the seam allowance: 241"

Now, in reality, I give myself about 6-12" extra just in case my folding is taking up more fabric than I calculated. That's probably a heck of a lot more than I need, but I figure better safe than sorry.

For the length of the skirt to touch the ground, I give myself at least 10" because I create a 4" double folded hem. My particular chair was lower in the back at the edge than it was in the front, so I hemmed AFTER I made the pleats.

Marking your fabric for pleats

Mark the top edge of your fabric every two inches. Then fold and press like this:

Attach skirt to seat and back

Note: I always glue baste things like skirts. Glue basting allows you to easily sew the skirt to the seat without fiddling with pins. It also assures you have the length exactly correct. I use a fine glue tip from Cristy Creates and Elmers washable school glue. Always use a dry iron to dry the glue in place.

1. Once you've finished pinning the pleats, sew a 1/4" seam at the top to keep them in place.
2. Right sides together, glue baste your skirt to the seat starting at a back corner and leaving 1/2" for a seam allowance. Mark 1/2" from end. This will be where you sew the two ends together.
3. Bring the basted piece to the machine. Starting at 1/2" away from the first corner, sew together using a 1/2" seam allowance. Stop when the end meets the starting point at 1/2". Then seam the two ends together. 

Hem your skirt

1. Put the cover on the chair and smooth. 
2. Smooth the skirt so it falls naturally around the chair, and place pins all the way around where the skirt touches the ground. 
3. Take the skirt off the chair and mark the line where the pins are using tailor's chalk or a frixion pen.
4. Mark 4" below that line and then another 4"
5. Trim at the second line down.
6. Fold four inches up twice toward the wrong side of the fabric. Glue basting each time helps keep it in place.
7. From the wrong side, top stitch your hem in place.

You are now ready to put your finished slip cover on your chair!