Open your thinkorswim account and navigate to the "Analyze" tab at the top the click "add simulated trades." The name is pretty self explanatory so as you guessed, this is where you would move on to the next step.
Next step is to type in your desired stock ticker symbol. This will give you the options list available for the company. Choose your desired expiration date and strike price. Here, you have the chance to create your strategy.
Here we selected random options for a bull call spread just for the sake of the example. As you can see we chose to buy one contract and sell another one at another higher strike price. Most people question how much money they will make from this strategy, this is where the next step will come in handy.
Navigate to the "Risk Profile" tab at the top of your screen and it will give you the graph of how much you can make or lose on the trade. You can go back and forth between the two tabs building different strategies until you're comfortable with the trade. The bottom left of your screen will give you your P/L risk profile. The purple line represents the date till expiration and the blue line represents the amount gained/lost as everything progresses.