Step 1 - Measure your mattress.
Measurement C is made up of the depth (or height) of your mattress plus some extra. The extra amount is determined by how much you want to sit under the mattress, plus a little for your elastic casing.
For a cot sheet, I did 10cms extra. The mattress was 12cm deep, so my final measurement C was 12cm + 10cm = 22cm. For a larger bed, you may want to allow more. OR you could measure the corner seam of your existing fitted sheet and add 2cms or so for the casing.
Step 2 - Cut your fabric
First you need to make a rectangle. The width of your rectangle is A + (2 x C), the length is
B + (2 x C). After you have cut your fabric of flat sheet into a rectangle, you need to cut a square from each corner, the size of which is C long, by C high. Discard these squares (or you may save them for another project if you like).
Step 3 - Sew the corners
At each corner where you have cut the squares from, you need to sew the two cut edges together to make a kind of box shape. Give yourself about a 6mm seam allowance. Finish the edges with an over-locker or if you don't have one, you could zig-zag the edges.
Step 4 - Make a casing for the elastic
All around the edges, you need to fold over about 1 - 2cms of fabric. Iron and pin this edge. You can also turn over the raw edge or you might choose to overlock your edges. Stitch close to the edge all the way around the sheet but leave an opening to allow you to thread through your elastic.
Step 5 - Thread the elastic
You will need some elastic, I would recommend something about 6mms wide, the length needs to be about (A + B) x 1.5.
Attach the end of the elastic to a safety pin and thread it through the casing. Join the edges of the elastic together. If you want a neat finish, you can sew the ends of the elastic together, but I usually just tie it in a secure knot. You could also stitch the casing closed, or not.
Voila! A fitted sheet!