A simple and virtually foolproof sewing project is a terry-cloth cover-up, made from two bath towels.

When shopping for the towels, remember that bath towel sizes vary a bit. Be sure to buy two that are big enough -- when sewn together -- to fit around you comfortably.

Then cut off the corners to form the "raglan" sholders.

Measure nine inches in from the corner and seven inches down. Connect these two points, cut out the triangle and discard. You may want to fold the towel first and make sure it fits before you cut.

Cut out the raglans on the top of both towels and hem all four raw edges. If your towels aren't too heavy, turn under twice and hem by hand or machine.

If the fabric is very, very heavy and if you have a zigzig machine, you might be able to turn it just once and keep the raw edge from raveling by sewing it down with a tight zigzag stitch. But test it first on the scraps.

Next, right sides together, sew up the side seams and press them open. The hemmed edge of the towel will form the hem of the garment. Depending on the towel, the hem at the top may be wide enough to use as a casing for the shoulder ties.

If not, carefully rip out the existing hem in each towel and make a new one, deep enough for a casing. Use a safety pin to feed cording or ribbon through both casings and it's ready to wear.

But you don't have to stop there. If you're using solid color towels, you can add a bit of embroidery -- anything from an elaborate garden of flowers to a simple monogram.

For design ideas, check the embroidery books at the library. While you're there, don't forget to look at children's books as well. Books for young children frequently contain the most marvelous drawings -- simple animals, butterflies, mushrooms and flowers -- all ready to be copied and used as an applique or embroidery motif.

Even if the towel is already patterned, I like to emphasize the motif with embroidery. The very simplest stitches will do: French knots in the center of a flower or a chain stitch to outline a stem.

A washcloth makes a wonderful pocket on this cover-up.

Q. In the fall of '77 I started six chair seats for a Christmas gift. I thought I bought more than enough wool for the project but, just to be sure, I purchased an extra box of background color yarn. After I had completed four seats, I realized I would not have enough to finish the background color of the other two and, what was worse, I learned that my original color was no longer being manufactured. Now what?

A. Thank you so much for you pathetic letter about running out of background wool for six needlepoint chair seats. It may be cold comfort to know that you are not alone in your plight, but I feel I must give you this handy rule for future reference: The most universal needlepoint yarn is called persian -- a 3-stranded, 2-ply yarn which comes in a beautiful range of colors and is usually cut in lengths of a little under a yard. Two full threads (6 strands) are enough to complete a square inch of No. 10, 12 or 14 needlepoint canvas in tent stitch.

However, many miscalculations can occur, so the best solution is to work your background color from the center out to fill in behind your design on all four sides. If you find you are running short of the background, you can add in another color (close to your original shade) and work with one strand of the old and one of the new mixed in the needle. The English work this way and create lovely striated backgrounds using two or three close shades of color. Instead of mixing the shades in the needle they work in short strips, blending the shades together to give a soft antique look, beautiful with the right design.