Sheeting fabric – you will need a piece 75" x 22" (190cm x 56cm) for a standard 30" x 20" (76cm x 51cm) pillow
You will also need a sewing machine, pins, a pair of cutting-out scissors, an iron, a long straight edge, a tape measure or steel rule, a T-square or a sheet of A4 paper and a “disappearing” felt tip or a water-soluble crayon in a colour that will show on your fabric.
Step 1: Cutting Out
Start by pulling a thread at the cut end of the fabric to give you a straight edge perpendicular to the selvedges. Cut along this line. Measure 22"/56cm (or whatever width you need) along each selvedge and mark those points with a "disappearing" felt pen or a water-soluble crayon. Measure and mark at various intermediate points too and then draw a line along a straight edge from one selvedge towards the other, to 75" (190cm) or whatever length you need. Draw in the short edge too, taking care to make it at right angles to the first line. You can use a sheet of paper to give you the right angle. Now cut out the rectangle.
Step 2: Hemming the Short Edges
At the other short edge, turn under 1 1/4” (3cm) to the wrong side. Pin near the raw edge and press the fold as before. Then turn the raw edge under by 3/8" (9mm), pin it and press it carefully, removing the pins as you work your way along the edge. Pin the hem – it should be about 7/8” (2 cm) deep. (Or you could miss out the second pressing stage and just go straight to sewing.) Sew close to the turned-under edge. We'll call this the opening edge/end.
Press both hems.
Step 3: Sewing the Side Seams
Creating the flap
Lay the fabric right side down. Fold the flap end back on itself (wrong sides together) to create a flap that is about 8" (20cm) deep, making sure the side edges are level with the underneath layer. Then fold the other end (opening end) back on itself too (wrong sides together again) so that the hemmed opening edge is level with the folded edge of the flap. The raw side edges of all 3 layers should be level with eachother. Now measure the length of the pillowcase, from the opening edge to the fold at the other end. It should measure the length you want the finished pillowcase to be, ie 30" (76cm) for a standard pillow. If it is not the right length then adjust the length of the flap as necessary and re-fold the bottom fold so that the opening edges are level again.
Sewing the first side seams
Now pin the two long side seams and sew them with a narrow (3/8"/9mm) seam, backstitching at each end. Finish off those ends by taking one thread through to the other side and knotting it to the other thread. Then do the same with the ends of the stitching of the two hems, first unpicking the stitches at each end so that the row of stitches ends just one stitch beyond the side seam. (This means that the stitches won't be cut when you trim the seam.) Trim the side seams to within 1/4" (6mm) of the stitching and then press the seams to one side – it doesn't matter which.
The second side seams
Turn the pillowcase inside out. Actually, it will be right sides in, because you have sewn the seams first with the wrong sides together. Then turn the flap inside out, so that it goes over the opening of the pillowcase. Looking at the pillowcase at this stage, there should be no raw edges visible on the outside. With the underside of the flap uppermost, the next layer down should be the wrong side of the underside of the pillowcase, and below that the right side of the top of the pillowcase. Gently push all 4 corners out with something like a knitting needle, then pin the side seams (through all 3 layers at the flap end). You may want to tack them and press them before sewing.
Sew both side seams taking a 3/8" (9mm) seam and backstitching at each end again. Finish off the ends and turn the pillowcase the right side out, making sure the flap ends up on the inside. Push all 4 corners out, press the seams and you are finished.