A window seat looks generous in photographs and slightly bleak in real life: a hard painted ledge where you meant to curl up with a book. The fix is a cushion cut to fit, and you can make one without owning a sewing machine. Fabric, foam and a reel of iron-on hemming tape will get you there in an afternoon.

This is the method to start with if you have never made anything before. The only genuinely difficult part is the measuring, and that costs nothing to get right.

What You Actually Need

• A tape measure, pencil and paper. The measurements are the whole job; the rest is assembly.
• Fabric. Upholstery-weight cotton, canvas or a heavy linen blend holds a crisp edge. Loose weaves fray and can be awkward to fuse.
• Foam. Look for medium-firm upholstery foam, 5cm thick for perching on, 7.5cm if you want to sit there for an hour.
• Iron-on hemming tape, sold as fusible webbing in most haberdashery aisles.
• An iron and a clean cotton pressing cloth, plus an ironing board.
• Fabric scissors or a rotary cutter, a ruler, and quilting clips or binder clips to hold folds.
• A bread knife with a serrated blade or an electric carving knife, for the foam.
• Optional: polyester wadding, a non-slip rug pad, and hook-and-loop tape.

To work out how much fabric you need, imagine wrapping the foam like a parcel. You want a piece twice the depth of the seat, plus the thickness of the foam, plus about 10cm for turning under, by the width of the seat plus 4cm. If the fabric has a large pattern repeat, add extra so you can centre the design.

Measure the Seat Properly

Window seats are rarely square, and old houses are worse. Take three width measurements — front edge, middle and back — and three measurements from front to back: left, centre and right. Write them all down rather than trying to remember them.

If the cushion sits inside a recess with walls on three sides, use the smallest figure from each set and take off another 5mm. A cushion that has to be forced into place will buckle at the corners and pop out whenever someone stands up. If the cushion sits on a flat sill or an open bench, use the largest measurement and let it overhang slightly.

Then decide on thickness. Deeper is not automatically better: 10cm of foam is hard to wrap neatly and can look bulky under a low window, swallowing the light you bought the window seat for.

Cutting the Foam

Mark your measurements on the foam in pen and check them twice. Lay the foam on a firm surface with the overhang supported, then cut with long, steady sawing strokes, keeping the blade almost vertical. A craft knife tends to wander and leaves a ragged edge; a serrated bread knife, used patiently, gives a cleaner line. If the seat is an awkward shape — a bay window with angled ends, say — many foam suppliers will cut to your measurements for you, which is money well spent.

For a softer look, wrap the foam in a single layer of polyester wadding and hold it in place with a light spray of fabric adhesive or a few pins pushed in flat. The wadding rounds off the sharp edges so they do not show through the cover.

Making the Cover: The Wrap Method

1. Pre-wash and press the fabric. Shrinkage after the tape has set will ripple every seam you have just fused.
2. Hem both short ends with a double fold. Turn the raw edge under by 1cm, press, turn it under again by 1cm, press. Two thin folds hide the edge and give the tape something to grip.
3. Fuse the hems. Tuck a strip of tape inside the fold, lay a pressing cloth over the top, and press firmly for around ten seconds without sliding the iron. Work along in sections. Leave it to cool completely before testing the bond — the tape sets as it cools, not as it heats.
4. Wrap the fabric around the foam with the right side facing out, bringing the two hemmed ends together along the front edge. Clip them in place temporarily.
5. Mark the side seams. While everything is wrapped, run a line of chalk or a row of pins down each side of the foam to show where the seam should sit. Take the foam out.
6. Fuse the side seams. Fold the fabric right sides together along your marks, sandwich a strip of tape between the layers, and press with the cloth. Turn the tube the right way out and press the seams open.
7. Slide the foam in, a few centimetres at a time, easing the fabric rather than forcing it.
8. Close the opening. Fold the hemmed ends under, tuck in tape and press — or attach hook-and-loop tape instead, so the cover can come off for washing.

Where It Usually Goes Wrong

The tape will not stick

Fusible webbing likes cotton, linen and poly-cotton blends. It struggles with coated outdoor fabric, heavy upholstery velvet and some slippery synthetics. Test a scrap first: fuse two offcuts, let them cool, then try to pull them apart. If the bond fails, change the fabric rather than the technique. A hot, dry iron works best; steam can make the tape bubble and lift.

The cover is too tight or too baggy

Fabric adds bulk at every fold, which is why the cushion needs roughly 5mm of breathing room on each side. If it is baggy, a layer of wadding around the foam will take up the slack without any unpicking.

The corners pucker

Trim the fused seam allowance back to about 1cm and press it open before turning the cover through. Bulk left inside a corner has nowhere to go.

Finishing and Looking After It

A cushion on a smooth painted sill will slide every time someone sits down. A rectangle of non-slip rug pad underneath solves it, or a few squares of hook-and-loop tape stuck to the cushion and the seat base. Vacuum the cover with an upholstery attachment rather than brushing it, and deal with spills straight away using a barely damp cloth and a little mild soap, working from the outside of the mark inwards so you do not spread the ring.

Rotate the cushion now and then if the seat is only ever used from one side, and keep it out of strong direct sun where you can, as fading shows first on the top edge. Foam is the part that eventually gives up; when it flattens, replace it and the cover will still fit.

Photo: Darya Sannikova / Pexels