Modeling the Side spoiler

The Side spoiler is modeled using three reference objects: Mudguard, Door_right and the Side rear. Again we use the same procedure as we did before. Duplicate the selected reference objects, then merge the duplicates to a single object, using the 'MergeO'  (merge objects) button. This results in a new object at the bottom of the hierarchy in the Select window. Rename the new object to 'Side spoiler'.

Selected objects are duplicated and then the duplicates are merged using the 'MergeO' button.

Switch to Point Edit mode and delete (use the Destroy button) all points, except the lower row. So you are left with a single row of points. From there we start building the Side spoiler.

Selected points are destroyed, leaving you with a single row of points..

Because the original three objects we duplicated (Mudguard, Door_right and Side rear) were separated by seams, we need to close these seams first before we can continue modeling the Side spoiler. Or else we get these seams (there are two) also in the Side spoiler. The seams are easily found, because there the points are very close to each other. Select the two points (also called 'vertices') in a seam area and merge them to a single point. Repeat that for the other seam.

Merge two vertices to a single point using the Merge tool.

From this point the modeling continues in the usual way, creating new edges and align the shape to the blueprint.

Match the blueprint

For the horizontal sides of the Side spoiler, we need to rotate the selected edges slightly to create a slight wedge shape (also see your reference images), like shown in the next image:

Rotate selected edges. Note the use of the pivot point which is located at the side of the Mudguard

Next we extrude extra edges for the air hole in the Side spoiler:

Edges added for the air hole

These edges need to fold to the outside of the spoiler. So, create new edges roughly and use reference images to make the correct adjustments.

The area indicated by the red arrow needs to be closed, point wise (using merge points)

Object closed again (vertices merged)

Go to Face Edit mode, select and extrude the faces just a little bit, like shown in the next view port screenshot. Be sure this time that you set the 'Leave Floor' option.

Set the Leave Floor option when extruding selected faces as shown in the next image...

Extrude selected faces

Then you need to close the rear end of the air hole. See next image: this face will not be used and can therefore be deleted:

This selected face can be deleted

Cut the Side spoiler using the Knife tool (not the Object knife!), to add an extra edge.

The selected faces are cut, along the red dotted line, to add an extra edge

Connect the faces from the air hole to the end piece of the Side spoiler. This may require some trial and error to get it really nice.

 
Connect the selected faces to the end piece                                  Screenshot of the connected parts

To finish it all, the lower edges are once more extruded and at the front of the spoiler some new faces need to be added to match the extrusion of the lower edges. Also check if you need to change edge sharpness here and there.

 
Extrude the lower edges                                                              and add some faces to the front side of the spoiler

And this is what we have now, and do not forget to save your work!

Screenshot showing progress until now!