Go to 'Windows' and select how many and what type of windows you wish to include into your plan. Then either click on 'Insert items' or drag & drop the windows icon into the plan.
Once your windows are added to the plan, you can click on each one and move it alongside the walls to the correct place.
To change how high from the floor the windows are positioned, click on the Settings and enter a new value for the parapet height. Select 'Apply changes to all existing walls' and click on 'Save'.
You can also change the parapet height for each window individually. Select the window and click on the i-button. Set the new parapet height as a value for the vertical axis Z.