Select NGon01 and Right-click on "NGon" in the modifier stack and then click Covert to: Editable Poly.
Go to Polygon sub-object mode and select the outer middle polygons that are inside the area of NGon02. Click the Settings button for Extrude under Edit Polygons roll-out in Modify Panel.
In the Extrude Polygons dialogue box, set the Extrusion Height = 21 and then click OK.
We have just opened our tutorial directory! Feel free to submit tutorials hosted on your website or other online tutorials that you think are worth mentioning.