Sewing a fitted sheet from a flat sheet is an easy project that can be completed in a few hours by a beginner. You can make your fitted sheet in much less time if you use a sewing machine. However, it can also be sewn my hand. You can purchase a new flat sheet for this project or use an existing one that you may already have at home.
Measure the mattress--width and length. Start your measurements from the side and bring the tape up and over the top and across the width to the other side of the mattress. Repeat the process to measure the length. Add six inches to each measurement.
Use a flat sheet that is the same size as your mattress or bigger. For instance, if you are making a fitted sheet for a twin size bed, then you will need a twin flat sheet to make your fitted sheet. Lay the flat sheet on top of the bed with the wrong side facing up. Center it evenly allowing an even amount to drape on all sides.
Make darts for all four corners. At the first corner, pinch the corner and bring the edges of the sheet together to form a half triangle. Pin in place, starting at the top of the mattress corner and adding pins all along the shape of the mattress corner until you get to the bottom. Repeat for the remaining three corners. When you're done, your corners with look like half-triangle flaps. These flaps will be removed in the next step.
Remove your sheet from the mattress with pins in place. Sew a seam along the pins of the triangle flaps for each corner. Remove the pins and cut the excess flap--about 1/2 inch--from the edge of your new seam. Repeat on all corners. Note: because you've used a flat sheet, the edges are already hemmed, thus saving you time and effort.
Cut elastic for the corners into 10 inch strips. Align one end of your elastic strip along the top of the raw edge of the corner perpendicular to the seam that you just made in the previous step. Pin the elastic in place. Repeat for the remaining three corners.
Place the elastic pinned piece-- beginning at the top of the first corner--under the needle of the sewing machine. Manually insert the needle down into both pieces. As you begin to sew--by pressing your foot on the sewing machine pedal--stretch the elastic fully taut and sew the elastic and fabric pieces together all the way to the end. Repeat for the other three sides. Flip your fitted sheet to the right side and place on the bed while matching corner for corner and tuck any excess under the mattress.