I hope vpelss is still around. You might try locating a copy of Random House Puzzle Maker's Handbook, first published in 1981 and reprinted in 1995. Authors Mel Rosen and Stan Kurzban recommend selecting a theme and then coming up with a long list of words and phrases that fit the theme. They should be at least nine letters in length. Avoid words with lesser-used letters and avoid words with long strings of consonants or vowels. Since the puzzle needs to be symmetrical, select two or three pairs of words of equal length. The theme words should be three or four rows or columns away from the grid's edge. Then put a black square at the beginning or end of each word. Add strings of two or three black squares protruding from the borders. Maintaining symmetry, add black squares to the grid in such a way that you don't wind up having to use overly long words. A grid will be easier to fill if you use words with alternating consonants and vowels. If one letter ends two different words, it should be an E or an S.
I hope that's enough information to get you started. Good luck!