Unless the pattern is so small and busy that it doesn't need to be matched, it must be a design that looks the same from each end of the scarf. In other words, if you hold the scarf by one fringed end with the other hanging down, it should look to an observer exactly the same as it would if you held it by the opposite end instead. This is because the fabric will be folded so that it runs down the front of the cushion, up the back and down the overlap. You should avoid a bold floral design because the flowers will be the right way up on the front of the cushion but upside down on the back. Tartans, stripes and checks work well, and fortunately they are common designs for scarves.
My scarf measured 68" x 14.5" (173cm x 37cm) which gave me 2 cushion covers measuring 13" (33cm) square. These fit nicely around 14" (35.5cm) square cushion pads - the pad needs to be 1-2" (2.5-5cm) larger than the cover to give a plump cushion.
I used a woollen scarf, but a linen or silk one would be just as good. Repurpose an old pashmina for a luxury look.
(To make 2 cushion covers)
A scarf with a fringe on each end
A length of 1/2" (12mm) wide bias binding (or 3/4" if the scarf is very thick), twice the width of the finished cover, in a matching colour
Matching sewing thread
4 large buttons, say 1" (25mm) diameter, to match the fabric
4 velcro circles or medium sized press studs
Scissors, pins, sewing machine, tacking thread and needle, iron
Step 1: Measuring and Cutting Out
Much depends on where the fringed end comes in the pattern. Look at the photo below - the overlap, with the fringe, needs to lie on top of the equivalent part of the pattern in the layer below, and that in turn will overlay the same part of the pattern on the cushion back. If your scarf has only, say 3" between the end of the last pattern repeat and the fringed end, then the overlap on the front of the cover will be only 3". That is really the minimum, any less and the opening at the top will not be properly covered.
Before you cut anything, fold the scarf around the cushion pad you are planning to use to check it is going to work.
Decide where to cut your scarf into 2 pieces. If its length is only about 5 times the finished width you want to achieve, then you will be cutting it in half across the middle - subject to what I have said above about the overlap possibly needing to be shorter than half of P. If it's much longer than 5 times, then measure in from the fringe at each end and cut at the appropriate point. Discard the middle portion, although you could make a corsage from it to decorate the finished cushion.
By way of example, my scarf measured 68" x 14.5" (173cm x 37cm), but the pattern wasn't quite central across the width (see photos), meaning that I only had 13.5" of usable width if I wanted the 2 blue lengthwise stripes to be equidistant from the centre. I wanted to keep as much of the width as possible to suit my 14" square cushion pads so I worked on the basis that I would only take 0.5" seams. This would give me a cushion cover 13" wide (and high). That tied in with 2 lengthwise pattern repeats, which measured 13". I therefore cut my fabric in half to give a 34" length for each cover, producing an overlap that measured 34"-(2 x 13") = 8" long, nicely within the 1/2 to 2/3 range.
Two things to bear in mind: firstly, your pattern repeat P will determine the height of the cushion, and its width if it is to be square; secondly, it would be best to allow at least 5/8" (1.5cm) seam allowances on each side, 0.5" is not really enough for fray-ey fabric.
Step 2: Sewing
(1st photo) Open out the bias binding and pin it along the cut edge of the scarf, right sides together and with the edge of the binding running along the edge of the fabric.
(2nd photo) Then stitch the binding to the fabric along the fold that is nearest the cut edge of the scarf. Press the binding upwards so that it protrudes beyond the fabric.
(3rd photo) Now fold the binding over the cut edge of the scarf and pin it down on the wrong side. It may be necessary to trim the scarf fabric seam a little, but be careful because it is a narrow seam.
(4th photo) Tack the binding in place then sew it (topstitching) close to the tacked edge. This binding will not be visible when the cushion is in use so concentrate on making it secure to contain the raw edge of the fabric rather than making it pretty.
(5th photo) The fabric must now be folded to create the overlap before sewing the side seams. Lay the fabric right side up on the table and fold the fringed end back on itself (right sides together) by the required amount to form the overlap. Make sure the pattern matches with the layer below. Pin the edges.
(6th photo) Fold up the other end so that the bound edge is just short of the first fold. Again, the pattern should match. Pin the edges. If a piece of the fringe is just inside the stitching line, pin it well inside to keep it from getting caught in the stitching. Any fringe pieces that are on the stitching line or in the seam allowance should be pinned out of the way in the seam allowance.
(7th photo) Stitch the side seams. (The right hand seam in the photo is wider than the left, I trimmed off the excess seam allowance afterwards.)
(8th photo) If you have trimmed the edges of the scarf to give you the width you need, then you will need to oversew the cut edges to stop them fraying. But this might not be necessary if the scarf is woollen and has been milled to make it slightly felted. On thicker fabrics the oversewing is best done on each seam allowance separately rather than with them both together, then the seam can still be pressed open and will be less bulky.
(9th photo) This is what it should look like when you turn it right sides out after sewing. You can slip the cushion pad in the opening at this stage to try it out, and turn the overlap over to the front of the cushion to close it.
Step 3: Closure and Finishing
With the cushion pad in place, mark where the Velcro closures or press studs need to go on the underside of the overlap and the cushion front beneath the overlap. The edge of each button should be about 1/2" from the lower edge of the overlap, so with 1" diameter buttons the centre of the velcro circle or press stud will be 1" from the fringed edge. Position them about a third or 2/7 of the way in from each side, wherever looks best. When you have decided where the centre of each button is to go, sew the press stud or velcro circle under it, taking your stitches right through the fabric for strength. Then sew the other half of the closure in the appropriate place on the cushion front, beneath the overlap.
Now sew the buttons on the outside over the ovelap, directly on top of the closure. If you have used press studs then stitch straight through the button holes and the press stud holes beneath.
Give your cushion cover a good press with a steam iron and then it is ready to use. But don't relax yet, you still have the second one to make!