Start With the Seat, Not the Fabric

Window seats are rarely square, and a cushion cut to a nominal measurement will bunch at one end and gape at the other. Measure the recess at three points across the width and three points front to back, then work to the largest figures. Do the same for depth if the seat tucks under a sill or sits beside a radiator, and make a note of anything that will foul the finished cushion: window handles, curtain returns, sockets, or catches on the underside of the sill.

Next, decide how thick the cushion should be. For a window seat, 7.5 cm to 12 cm of foam works well. Ten centimetres is the usual sweet spot — deep enough to sit on comfortably, shallow enough not to swallow the room. Order your foam cut a fraction larger than the recess so it fills the corners, and cut the cover panels to the foam size plus seam allowances. Fabric has give; foam does not, and that slight compression is what gives a box cushion its plump, tailored look rather than a loose slipcover feel.

Working Out the Cutting List

Sketch the cushion as five pieces: a top panel, a bottom panel, and boxing strips that run around the sides. Allowing a 1.5 cm seam allowance throughout, the sums are straightforward.

• Top and bottom panels: foam length and width, plus 1.5 cm on every edge.
• Front and back boxing: the width of the top panel plus 3 cm (two seam allowances), by the foam thickness minus 1 cm for a snug fit, plus 3 cm.
• Side boxing: the same treatment across the narrower measurement.
• Zip: roughly 5 cm shorter than the back boxing strip, so the zip sits neatly between the corners.

As an example, a seat recess of 120 × 50 cm taking 10 cm foam gives top and bottom panels of 123 × 53 cm, front and back boxing strips of 126 × 12 cm, and side strips of 56 × 12 cm. In a 140 cm wide upholstery fabric that lot fits into about 1.6 m of length, so allow 2.5 m to cover piping and any pattern matching. Add a full pattern repeat for each cut piece if the design is large or directional.

Make the Piping First

Piping is what separates a homemade cushion from a made-to-measure one, and it is easier than it looks. Cut bias strips 4 cm wide — bias, not straight grain, so the piping curves smoothly around the corners — and join them on the diagonal with a 6 mm seam. Press the seams open, then wrap the strip around a 4 mm cotton cord and stitch close alongside it using a zipper foot, with the needle as near the cord as the foot allows.

You need piping for the top edge and, if you want a really crisp result, the bottom edge as well. That is around 7 m for our example cushion, so buy a little more cord than you think you need; running short halfway round is a miserable experience. Trim the piping seam allowance to 1.5 cm before attaching it, and snip into the allowance at the corners so it turns without puckering.

Fit the Zip That Suits Your Seat

A lapped zip is the most forgiving option for a first attempt. Split the back boxing strip into two pieces, making one 4 cm wider than the other, and press a 2 cm turning along the wider piece. Place the zip face down on the narrower piece, stitch, fold the wider piece over to hide the teeth, and topstitch through all layers. Open the zip before you go any further — there is nothing worse than finishing a beautiful cover with the zip closed and the foam waiting on the table.

If the seat is in a hard-working spot, consider a concealed zip set into a seam instead. It looks cleaner but demands more accuracy, and a lapped version can be unpicked and replaced far more easily if a zip ever fails.

Assemble in the Right Order

Join the boxing strips into a continuous ring, right sides together, and press the seams flat. Sew piping to the top panel, clipping the allowance at each corner, then pin the boxing ring to the top panel with the piped edge sandwiched between. Work around one side at a time, pivoting at the corners rather than trying to sew round them in one sweep. Repeat for the bottom panel and the remaining piping.

Before turning the cover through, grade the seam allowances — trim the piping allowance to about 8 mm and the boxing allowance to 1 cm — so the edges lie flat rather than lumpy. Clip any curves, then turn the cover right side out and press the seams from the inside, using a pressing cloth over the piping so you do not flatten it.

Fill, Fit and Keep It Looking Sharp

Wrap the foam in a layer of polyester wadding before inserting it; the wadding softens the edges, fills out the corners and lets the foam slide in through the open zip with far less swearing. Ease the cover over the foam, working from the corners inwards, then close the zip and smooth the boxing flat with your hands.

To keep the cushion looking tailored, rotate it every few months so the wear is even, and plump the foam by pressing down on the corners. Vacuum the fabric with a brush attachment rather than scrubbing it, and treat spills by blotting towards the centre with a clean cloth. A removable cover earns its keep here: when it eventually needs a proper clean, the cover comes off and the foam stays exactly where it belongs.