Contemporary families find it easier to skip flat sheets and use comforters and a fitted sheet alone. This simplifies bed-making, and it alleviates the frustration of a flat sheet gone wayward during the night. But it also leaves the puzzle of what to do with all of those unused flat sheets from bed sets. It is relatively easy and takes very little sewing skill to convert a flat sheet into a fitted sheet. And the extra fitted sheet will come in handy. These instructions are written for a full-sized bed but can be adjusted to convert sheets of any size.
Trim the flat sheet of all hems. However, if you think you may need all available fabric, remove the stitches and press out all hems.
Remove all bedding from the full-sized mattress, leaving it bare.
Spread out the flat sheet across the mattress so excess fabric falls equally from both sides. Adjust the sheet so the fabric also falls equally from the head and foot of the mattress.
Mark each corner of the mattress on the flat sheet with a straight pin. Most mattress corners are rounded, and a pillow-top mattress may have considerable lift as well, so pull out the sheet from the mattress at each corner and mark the approximate point where the mattress edges would meet if they were perfectly squared. Then move the pin 1/2 inch closer to the flat sheet corner to make the fitted sheet just a bit bigger.
Measure from the point of the corner to the edge of the fabric on each side to make sure the distances are equal. Also measure the sheet's overhang at both the head and the foot of the mattress to make sure they are equal. Adjust the corner markings, if needed.
Measure the sheet overhang on all four sides of the mattress. You'll need to trim the sheet so the overhang is equal on all four sides. Determine how much you'll need to trim and where, then remove the sheet from the mattress and lay it out on a flat surface, such as a tabletop. Do any trimming there to avoid accidentally damaging the mattress.
Fold each corner of the sheet fabric to the point marked by the straight pin. Do not remove the pins yet.
Working with one corner at a time, fold back the sheet's corner to a point along the sheet's side edge to form a right angle with the point marked by the straight pin. Use the weave of the fabric to guide you. Press this fold in place with the iron. This pressed fold line marks where you will stitch the seam for the fitted sheet corner.
Stitch together the four corners of the sheet along the fold lines that you marked by pressing the folds. Trim away the excess triangles of fabric.
Slip the half-constructed fitted sheet over the edge of the mattress. Take note of how much overhang you have. Remove the sheet. Round off the corners of the sheet with scissors, starting about 12 inches from the actual corner on both sides. If you have a small overhang, you won't be able to round off as much of the corner as you would with a larger overhang. But you want to round it off as much as you can to make a continuous hem around the complete periphery of the sheet.
Pin under a 1/4-inch hem all around the periphery of the sheet. Press.
Pin under another 1/2-inch hem all around the periphery of the sheet. Press again.
Stitch the hem in place, leaving a 1-inch gap through which you will thread the elastic.
Thread the braided elastic into the hem, securing one end as you work the elastic through the hem and back to where you began. The actual amount of elastic you'll use will vary, depending on how much fabric overhang you had to work with. Use enough elastic to pull in the corners of the fitted sheet without making it so tight that it's in danger of tearing. Pin the ends of the elastic together with a safety pin and test the sheet on your mattress before finishing off the project. Make any necessary adjustments. Stitch the elastic ends together. Finish off the hem.
If you cannot find a single length of elastic long enough for the entire sheet, you can sew two lengths together for an appropriate length. Pin one end of the elastic to a safety pin to thread it easily through the hem.
Make sure there are at least 5 extra inches of fabric overhang (minus the hems) past the bottom of the mattress all the way around the sheet, especially on both foot and head sides of the flat sheet, before attempting this project. Some flat sheets are more generously sized than others. If the sheet is not long enough, you'll need to either add fabric to each end or consider another project, such as making bedroom curtains instead from your flat sheets.