A microwave for Christmas and you haven't a clue what to do with it. There it sits in the corner of the kitchen, cool, compact and computerized. It's nice, yes, but you really would have preferred diamonds.

Microwaves are so often bought in a fit of efficiency, and then left to sit, taking up space, their biggest contribution being to defrost orange juice, heat cold coffee and bake an occasional potato. But put these instruments to use, and you will not only save time for yourself but also save money on your energy bills.

Microwaves cause heat by friction. And contrary to what many believe, they do not leave any radiation in food. The microwaves penetrate food from all directions and cause vibration in the plus and minus particles, which in turn generates heat and cooks the food.

Various reasons are cited for snubbing the microwave. Health comes to mind, but no one can agree on that subject. Laziness is mentioned; some are just too lazy to figure out how it works. However, fear is the most popular; not only of the dangers to their health, but too many fear that their potatoes will explode or that their pork chops will shrivel up to bacon crisps. If they follow these few guidelines from "Basic Microwaving," by Barbara Methven and "The New Revised General Elecric Microwave Guide and Cookbook," however, things are sure to run quickly and smoothly.

Prick or pierce foods with membranes or tight skins such as egg yolks, oysters, chicken livers and potatoes to avoid explosions.

Never microwave an egg in its shell; steam builds up inside and the egg will burst.

Do not microwave bottles with narrow necks, they may shatter.

Foods that are higher than 3 inches may have to be shielded if microwaved for long periods.

Place thin parts toward the center of the dish where they receive less energy. Thin parts of uneven foods cook faster than thick parts.

Remember that the larger the amount of food you place in the microwave, the longer time it will take to cook. Five potatoes will take 16 to 20 minutes, one potato will only take 4 to 6 minutes.

Do not salt tops of vegetables before microwaving; this causes darkened, dried-out spots.

Do not use conventional meat or candy thermometers. They may be used to check temperatures outside the oven, but only microwave thermometers can be used inside.

Do not microwave foods in metal or foil containers; metal and foil will reflect microwaves and prevent even heating. Electrical sparks can occur when two pieces of metal are placed within 1 inch of each other.

To check to see if a container is microwave safe, measure 1 cup of water in a glass cup. Place in oven on or beside the dish you are testing. Microwave 1 minute at High (10). If water becomes hot, dish is microwave safe. If dish heats, it should not be used for microwaving.

If this list seems limiting, microwaving is not. Forty-five to 50 percent of Amercian households now own a microwave, proving that single cooks and family cooks alike have discovered its varied potential. Leftovers can be warmed without drying out, vegetables can be cooked with no water added, thus fewer nutrients lost, and the fat of fried bacon can be slurped up by a paper towel. Use it to speed up conventional cooking by melting butter quickly to use in baking or in sauces. Fillings and sauces for crepes can be microwaved before the product is finished in a crepe maker or skillet. Brown meats in a skillet and transfer them to a microwave-proof dish for a reduced amount of cooking time. Even get more juice from lemons by microwaving 20 to 25 seconds at high before cutting and squeezing.

And you do not need to buy one special microwave dish to employ your microwave. Oven glassware such as measuring cups, baking dishes, casseroles and mixing bowls, glass-ceramic (pyroceram) and a lot of regular dinnerware are all suitable for microwaving. Even handmade pottery, porcelain and stoneware are effective.

New cookbooks aren't needed. Many recipes can be converted to microwave cooking. Follow a similar recipe that comes with the microwave oven and remember to:

Reduce the amount of liquid in the recipe to about 3/4 as liquids do not evaporate when microwaved. If necessary, you can add more during microwaving.

Use slightly less seasoning, especially with strong flavors. You can correct to taste later.

Reduce the amount of added fat. Fat attracts energy and slows cooking of other foods.

Remember to let the dishes stand a moment before eating, allowing them to finish their cooking process.

Arrange foods in a ring in a microwave, if possible, so that all sides are exposed to the microwaves.

Reduce conventional time to 1/4 or 1/3.

Remember that some conventional recipes just can not be converted. Fried foods such as chicken, hash browns or french fries become soggy, and crusty foods like popovers, pancakes, pizza and two-crust pies remain crustless.

Give yourself a present of time this new year by using the microwave to its fullest potential. If you have butter, flour, salt and pepper in your cupboard, dash through the express lane and prepare this creamy dish in less than 20 minutes.

EXPRESS LANE: macaroni, dry mustard or nutmeg, milk, gorgonzola, fontina, mozzarella, parmesan, paprika 15-MINUTE 4-CHEESE MACARONI (6 servings)

7 ounces elbow macaroni

3 tablespoons butter

3 tablespoons flour

1 teaspoon salt

1/2 teaspoon dry mustard or pinch nutmeg

1/2 teaspoon pepper

1 1/2 cups milk

3/4 cup gorgonzola, crumbled

1/2 cup fontina, grated

1/2 cup mozzarella, cut into 1/2-inch cubes

1/4 cup parmesan, grated

Paprika for sprinkling

Cook macaroni conventionally. Drain; set aside. Melt butter in 2-quart casserole on high power, 40 to 55 seconds. Stir in flour and seasonings until smooth. Microwave 30 to 45 seconds until heated. Blend in milk. Microwave 4 1/2 to 5 1/2 minutes until thickened, stirring every minute. Stir in gorgonzola and fontina until melted, microwaving 15 to 20 seconds, if needed. Mix in macaroni well, stirring in the mozzarella. Sprinkle with parmesan. Microwave 5 to 6 minutes to heat through, stirring once. Dust with paprika. Adapted from "The Silver Palate Good Times Cookbook," by Julee Rosso and Sheila Lukins (Workman Publishing, 1985) and "Basic Microwaving," by Barbara Methven (Microwave Cooking Library, 1978)