Log in to your TD Ameritrade account.
Locate the stock you would like to purchase. This can either be done by entering the stock symbol in the "Get Quotes" field at the top of the screen or using the "symbol lookup" tool, which is also located at the top of the screen.
Click the green "Buy" button on the stock's page. This will fill in the order form at the bottom of the page.
Enter the quantity of shares you would like to purchase and set the order type to "Stop limit."
Fill in the price fields. The "Price" field should contain the price you would like the order to activate at, and the "Act price" (Actual price) field should contain the upper limit you are willing to pay per share.
Set the expiration date for your order. Click "Review order," and if everything is to your liking, click "Place order."