How do I declare commission from real estate if my broker does not offer a 1099?

It depends. Are you a realtor/sales agent. If yes, then this information will flow on Schedule C. 

Please follow the steps below:

1. Click on Tax Home from Left Side Menu.
2. Click on Income & Expenses from the main screen.
3. Scroll down till you see the section "Self-Employment". Within this section you will enter all your income and expenses.
4. Enter all the relevant information and enter all your business information (Even if you are an independent contractor, you are considered self employed).
5. When you enter all the info, you will get a summary of the business information. Scroll down and click on Looks Good.
6. Then it will ask you how you received income. There will be an option for "Other Self-employment Income". Select that option and click Continue.
7. Then you can enter why you received that check and the amount and proceed ahead.