How to use a protractor if you can't see what you are measuring? I have this protractor:

It seems to be the same or similar as this one: Robinson 5001# Stainless Steel Protractor
RBS-R-5001 but there are no instructions there.
The main problem with this protractor is that the 'origin' swivel point can't be accurately placed over the object you are trying to measure because you can't see where it is. For instance if you put it over a paper image of a triangle to measure an interior angle you can line up the tip of the protractor (small detent) but not the swivel point.
Also I don't see how you could use this e.g. to measure the interior angle of a real object as you would with a bevel gauge. So it does seem intended to be used for flat measurements, I just can't see how.
The angle between the ruler part and the the semicircular part is the same as the angle indicated on the scale.  This is because the centreline of the ruler is parallel to its edge.
In more detail:

Image from Wikimedia Commons user Krishnavedala
Consider line a to be the top edge of the ruler in the picture and line b the centreline.  Line t, the transversal, is the base of the semicircle.  The top right θ where a and t meet is the angle you want to measure.  The bottom left θ is the angle the pointer makes with the semicircle.
This is a useful protractor for setting an angle or measuring a physical object with some thickness. For work purely on paper I'd prefer the clear plastic sort, marking in pencil and then using a steel rule for cutting if necessary.
Say you want to measure the interior angle of the parallelogram shown in freehand blue in this picture (imagine it drawn on paper under the ruler).

You align the protractor as shown, so the edges highlighted in thin red  run along the lines you're interested in.  Then the other angle in red is what you read off.  This is a copy of the first red angle, rotated and enlarged (hence thicker), but not distorted.  You can see that this runs through the pivot and point to the pointer, aligned with its groove.