Want to be a fashion designer? There are two routes. One is to sketch or make the most beautiful clothes you know how, refine them each season, and present them when your time comes up on the fashion calender.

The other is to survey what everyone else is doing, put those ideas through a bit of modification and editing and send them down runaway as your own.

In the fashion business today there are plenty of editors or modifiers, including some big names, who call themselves designers. And them there are there the originators - Sonia Rykiel, Goeffery Beene, Stephen Burrows - and some lesser-knows.

Koos van den Akker, whose collection this week marked his 10th year in this country, is taken with the traditional costumes of his native country, Holland. His special skill is artfully applying appliques on fabric for a college effect. Marlo Thomas is a fan, as are Cher, Lola Redford, Mrs. Oscar Wyatt and Mrs. Tony Bennett.

Van den Akker, who says he remembers making costumes for a church production of "Gone With the Wind" in crepe paper when he was 11, worked in display in a large departmetnt store in The Hague, then as an interpreter at Gallerie Lafayette, the Paris department store.

He put in a stint as an apprentice at Dior, and six years making custom dresses for his own shop in Holland where he got lots of attention but didn't make much money. With a missionary visa (immigration had said there was no need for more artists) he arrived in New York. After several Seventh-Avenue jobs, he struck out for his own boutique again, and for three years has had a successful shop and wholesale operation, selling to, in Washington, Neiman-Marcus and Saks-Jandel.

This may be his best collection ever, with the collage-like technique applied to Chanel-style suits in tweed. There is a fabric mix in the lining and facing of a jacket to match a blouse, and a third fabric and color scheme introduced as edging. Some versions for evening have a row of rhinestones as well. An unexpected combination uses fur and fabric in an extraordinarily successful lumber jacket shape.

"There is no fad in my clothes. In two years or in 10, I will still be doing what I want to do. i can't help it," he says. His suits run about $1,5000, the fur jackets about $1,3000.

Harriet Winter calls herself "Mrs. H. Winter" because , says, "I'm married and I'm proud of it." Her husband, her business partner, gave her $2,200, an assistant and an iron to go into dress designing six years ago. Before that, they had been in the antique clothing business under the name "Yesterday's News," selling to musicals like "Hair" and "Grease" as well as tot several stores across the country.

Her clothes are characteristically soft and unlined , with seams on the outside and with shape achieved through deft use of shirring, drawstring and cut. Shapes are refined, even repeated from the season to the next with new things added. This time she has even ventured into fur, rabbit, which she has grooved to look like corduroy and also done as trim in jackets. They are very good.

Why the exposed seasm? "We live wiht line and texture all day," says Winter. "Even the leaves don't hide their veins. The seam has a function in clothing. Why deny it - why not use it?"

She's made a subtle silhouette change to wider shoulders this season by building up the sleeve or using the outside seam to heighten the illusion of width.

Her clothes for fall will strat at $60 for a blouse or skirt. Bloomingdale's and Saks Fifth Avenue carry them.

Gil Aimbez has trouble confinig hinself to doing just a few kinds of things. He started with sportswear, included a few dresses and last year added a dress division. This time he has added a coat division, called Snafu.

One of his clever ideas is a coat in three layers. The bottom layer is a long, below-the-calf coat, and the top layer a vest. It's not a bad design, since you can add parts to fit the weather. Each layer is a different color, but they're all close natural tones - in this case clay, acorn and twig. He uses the same triple layering concept in a tunic.

"He's really a couture designer," says Dawn Mello of Bergdorf Goodman, praising his collection after the show. Mimi Liebeskind of Ann Taylor agrees. "He's got wonderful details. It is time to get back to neatness and he knows how to do it.

Not at couture prices, fortunately. For fall the dresses and sportswear will start at $70, the coats at $200. Gil Aimbez designs are carried in Washington at Woodies and Hecht's as well as Ann Taylor.