Seat Covers with Ties and Ruffles

The fabric on our chairs was wearing through. I wanted covers that I could take off and wash, that would look nice and stay on securely. I opted for four ties to attach to legs for a secure hold, and a pretty ruffle (or fitted side panel) to jazz them up a bit.

Our chairs (shown) are:

18" at the widest (front) end
15" at the opposite (back) end
18" deep (front-to-back)

Units:
All units are inches.
For example, $\,w + 1.25\,$ would denote $\,w\,$ (in inches) plus $\,1\frac 14\,$ inches.

Seam allowances:
All seam allowances are 5/8".
Thus, two seam allowances is $\,1.25\,$ inches.

Cut seat top:
Let $\,w\,$ be the widest chair width (side-to-side, in inches).
Let $\,n\,$ be the narrowest chair width (opposite end, in inches).
Let $\,d\,$ be the chair depth (front to back, in inches).

Cut cover fabric (one piece for each seat cover):
$\,(w + 1.25)\,$ (wide width) by $\,(n + 1.25)\,$ (narrow width) by $\,(d + 1.25)\,$ (depth)
This allows for a very-slightly-overhanging top fit, plus 5/8" seam allowances.
Of course, the narrow width should be centered across from the wide width;
go in $\,\frac{w-n}2\,$ from the edge.
If you're cutting more than one cover from wide fabric,
you may want to use the layout shown below.
However, this layout may not be appropriate if your fabric has a one-directional pattern or nap.

Round/fold back corners:
Slightly round (by cutting) the two corners of the widest side (front of chair) to fit the chair shape.
This makes the ruffle/side panel attach more easily and look nicer.

Fold a triangle in slightly (wrong sides together) on the two remaining corners (back of chair).
Stitch close to edges.
This reinforces the back corners, and gives a better fit around the two chair back posts.

Cut ruffle/side material:
There are two ruffle/side pieces: one goes along the back, the other along the remaining three sides.
The two splits between the ruffle/side pieces are for the chair back posts.
One edge of the ruffle/side is finished; the other edge is sewn to the seat fabric.

Let $\,f\,$ be the desired finished height of the ruffle/side (from the seat top, hanging down over the side).

Let $\,m\,$ be the ‘ruffle multiplier’:

for a very full ruffle (lots of gathers), let $\,m =3\,$
for a mild ruffle (fewer gathers), let $\,m = 1.5\,$
for no ruffle (just a straight side piece), let $\,m = 1\,$; for thick upholstery material, this works best

Cut the ruffle/side material, piecing as needed:

3-side piece: length $\,m(2d + w) + 1.25\,$ and width $\,f + 1.25\,$
back piece: length $\,mn + 1.25\,$ and width $\,f + 1.25\,$

Finish edges of ruffle/side:
For each ruffle/side piece:

Fold over about 1/4" on one long edge and zig-zag.
Fold over again about 1/4" and zig-zag to complete the finished edge.
Repeat (fold/zigzag, fold/zigzag) for one of the two short edges.
The remaining short edge is finished when you're almost done attaching the strip to the seat, since it's difficult to predict exactly the correct length.

Gather the ruffle strip (for $\,m > 1\,$):
Here's how I deal with gathering long strips of heavy material:

Cut a piece of thread a little longer than the desired length of finished ruffle.
Sew several stitches 5/8" in on raw end of strip to secure.

Hand-sew with long stitches for about a 10" length through the raw edge of ruffle;
adjust gathers to make the gathered length $\,\frac{10}{m}\,.$
For example, with a multiplier of $\,m = 1.7\,,$ $\,10\,$ inches gets shortened to $\,\frac{10}{1.7} \approx 6\,$ inches.
Stitch several stitches in place to lock.

Repeat in ten inch sections to the end of the strip.

Attach the ruffle/side strip:
Attach the three-side piece first; back piece last.

Start with the end with the finished short edge.
Machine-sew ruffle/side to seat, right sides together, with a long stitch.
Stop close to the other short edge, cut to the needed length, finish the other short edge, then finish attaching.
Machine-sew again, very close to first stitching, with a shorter stitch length.

To keep ruffle/side from popping up, and to give a finished appearance:
Fold the seat and ruffle/side wrong-sides together, and stitch close to the top edge around the entire seat.

Ties:
Cut four pieces of ribbon or cord, 20" each.
(If the cord frays, light the edges to seal.)
Machine-sew the center of each 20" piece to the seam allowance of each front corner.
Cut each of the remaining two 20" pieces in half (seal edges as needed);
machine-sew to the inner-bottom of the four back ruffle/side pieces.

If you want the bottom of the ruffle a little tighter:
Sew narrow elastic to the bottom of the ruffle/side, stretching as you sew, all the way around.
If desired, fold over to hide the elastic, and stitch again.