I'm self employed, with my own mug and t-shirt making business. How do I estimate my labor costs that are incurred when making my own products?

You don't have to. Your labor costs as the owner and sole proprietor of the business will be a function of your business profit. From there, you will pay taxes and Self-Employment taxes on those profits.

Unless you have other employees, there is nothing to enter for labor costs or salaries.

If you have additional questions or details regarding this, please feel free to post in the comments for further clarification.

You never get a tax deduction for the value of your own labor.  Your labor is what makes the profit -- you sell the product for more than the cost of the raw materials because your labor adds value.   If you pay someone else for labor, that is a cost.