You can use the following command to open the Quick Fix window:
:copen
By default the Quick Fix window will open with a height of 10 lines, meaning it can fit 10 lines of text. You may want to specify the height of the quick fix window when you open it, especially if you are working with limited screen space. This can be done by prepending the command with a height.
:5copen
This will open the Quick Fix window with a height of 5 lines.
See :h copen for more details.