Disclaimer: Always use a demo account first to run your strategies!
If you are ready to put your strategy into action on your demo account, the first thing you need to do is open a symbol chart. Find the symbol you want to trade in the Market Watch section, right-click, and press Chart Window.
A chart window will open up. Navigate to your strategy in the Expert Advisors section, click it, and drag it onto the chart. You'll see the following window, and you check the checkbox that says Allow Algo Trading.
Press OK.
The Expert Advisor is now loaded onto the chart, but will not be trading as long as Algo Trading is not turned on fully. Navigate to the top toolbar and find the Algo Trading button. If it's off, it's a red button like you see in the screenshot below.
When you press it, it turns on and turns into a green play button.
You will also notice that the little hat in the top-right corner of your chart window will turn from grey to blue, indicating that your strategy is now active and will be taking trades.