17. Continue extrusions the work, use shift + drag mouse, and after you can collapse the correct vertexes.
18. Here, we have a problem: the edge breaks the "Edge-Loop", so we have to fix this edge again. Select the edges as image 030, click the button COnnect and our Edge Loop is OK! Have the vertexes organized always.
19. It will be a good idea to use the snap 3d, with it you can unify a vertex and its correspondent.
20. here, we have to keep extruding.
21. Ok, here is the model.
22. Select the sub-object Polygon, make a selection of the faces as image 033 and click Hide Selected.
23. You can work with shortcuts, below you can find my suggestion for shortcuts, or you can configure your shortcuts in Menu Customize - Customize user Interface - option Keyboard.
Extrude = Shift + e
Bevel = Shift + b
Outline = Shift + o
Inset = Shift = i
Chamfer = Shift + r
Connect = Shift + n
Weld = Shift + w
Collapse = Shift + c
Hide Selected = Shift + h
Repeat Last = Ctrl + r
Ring = Alt + r
Loop = Alt + l
24. In the frontal viewport, select the edge as the next image and click Ring (Alt + l).
25. Click on Connect (Shift + n), and after edit the new edges.
26. Select the inferior edges and extrude it (SHift + e) (Image 036).