You can deduct worthless stock only in the tax year it becomes completely worthless. This normally happens when the corporation files for bankruptcy, stops doing business, and has no assets. Financial difficulties won't make a company's stock worthless unless there's no hope that the company will pull through.
Enter a worthless stock like any stock sale but with a sales price of zero and the word worthless in its description. Enter the correct cost or basis, date acquired, and December 31 as the date sold.