To place a standard rectangular window in an existing wall, follow these steps:

1. In the Wall Drawing component tab, select the Window tool:
   
   
   
   
2. Move the window into the drawing area. The window snaps in line with existing walls.
3. Use the input fields in the properties box to specify the dimensions. To place it at a certain distance from the end of the wall, use the Left or Right fields.
   
   To determine Right and Left of the window, imagine you are standing facing the inside of the window, i.e. on the opposite side of the blue arrow. If you type a distance into the wrong cell, press the Spacebar to unlock the field, then press Tab to move to the correct line.
   
   
   
   
4. When it is in the desired position, click to place one end of the window. A properties box opens.
5. The width of the window can be determined either by entering a value in the properties box, or by moving your cursor to the desired width in the drawing view. Once the width is set, left-click or press Enter. 
   
   When placing a window in a curved wall, the max width is 3 meters, or 9'-10". The max thickness is 3 times the wall's thickness, except for thin walls where the max thickness is 20 cm, or about 8 inches. The thickness can only extend on the outside of the curved wall.
6. Clicking the green double arrow will toggle the window in and out, while clicking the blue double arrow toggles it right and left (hinge side). 
7. After placing a window you can still change its width, height, and other values by selecting the object in the drawing and entering new settings in the properties box that appears.
8. You can also stretch the component manually after placing it by clicking and dragging the different yellow snap points.

In the 2D view, a selected window looks like this:

The triangle shows the window direction and should always be on the outside of the window, pointing inward. In the 3D component tab, you can add the Window light tool to have sun shine through your window when rendering. The window direction will also determine the opening side if you set an Opening angle for your window.

You can turn off the direction indicator in the Wall Settings dialog: