In my experience, a twin sheet makes one table cloth (for a smaller table like mine) and four 17" X 17" napkins. Here is how I do it:
1. Picking a sheet: Picking 100% cotton wovens is your best bet. No stretch!
2. Now that you have it, you need to wash it. Really well. On hot if you can... and heated dry. You want anything that will happen to this puppy to happen BEFORE you put a bunch of time turning it into your masterpiece. This has been an issue for me before, so don't try to skip this step. Sometimes you find sheets that look brand new! How exciting! Except that they ARE brand new, and when you wash them for the first time they will shrink unevenly and make your napkins look like crap. Nothing stinks more than spending a bunch of time on a cute project just to have it ruined the first time through the wash.
3. Iron your clean fabrics.
4. The measuring. Measure your table and your current 'favorite' cloth napkin. I like my table clothes to hang down about 8 inches on either side of the table, so I use my table measurements and add 16 inches to the width and length. My favorite napkins are all 17 by 17 inches.
5. Then I cut. I cut the table cloth first, then the napkins. I usually cut a 18inch long strip, and then fold the napkin over, making a perfect square with my fabric and cutting it off on that edge. This leaves a nice seam allowance and makes cutting easy. Think of the way you used to fold an 8X11 piece of paper to make a square. That is the same concept.
6. Then comes the hemming. Iron down a 1/2 inch of fabric all the way around your tablecloth. Then go around and iron it down again, making a double hem.
7. Hemming the napkins is the same, except you use a 1/4 inch double hem if you can. This takes a lot of starch. Starch is your friend. Don't be afraid to use it.
8. Then stitch the hem down. If these are starched and pinned, they should go easy and smoothly. Keep your needle down on the corners and turn that way, making a folded square corner.
A couple of notes:
I tend to be picky about brands when it comes to shopping for thrifted linens. Name brand linens are usually better made than the off brands. They are also usually better fabrics as well... being mostly cotton or all cotton of good quality. I avoid the sheets with any sort of Lycra for this project. The stretch makes it harder to get square corners and strait seams... even with an iron and starch. However, if you find a print you LOVE and you can't find the tag saying what it is made of, give it your best guess. Worst case, you have a bunch of fabric that you like for another project that you paid next to nothing for. You really can't go wrong