After teaching creative writing in pricey one-on-one sessions for 36 years, writer-publisher Sol Stein came up with what he describes as a much more cost-effective "method of cloning myself." In 1989 he encapsulated his lesson content in a software program called WritePro, which, says Stein, will assist you with those niggling novelistic details like plot, character development, and provocative dialogue. A six-pack of lessons retails for $120 for the IBM version and $140 for the Mac version. Stein's company, WritePro Inc., based in Scarborough, N.Y., distributes nationally through bookstores and computer retailers and has set up a toll-free number for phone orders. WritePro has few competitors and so far has racked up $1 million in sales, enabling Stein to easily recoup his initial investment of $15,000.