Have a look at my video here.
https://youtu.be/mc_Ow7KqLlg
I cut out a circular block about 3" diameter then using a bandsaw with the table set at an angle (you can use a scroll saw or hand saw) I cut slots into it any number from 3 to 6 is my normal number according to the power required and insert metal blades (cut using tin-snips) into the slots. they work really well and are very efficient.
Notes
The angle of the blades is not critical nor is the length or surface area.
For simpler whirligigs with one simple movement I use 3 or 4 blades for more complex motions I use 5 6 or 7 blades, I have even tried a 12 blade propeller out!
I normally make the hub circular but a square or other multi sided shape works just as well.
make sure you give the propeller some nice thick coats of paint, I even put a couple of coats of yacht varnish on top.
Any thin alloy or tin (scrap) will do for the blades, my last ones were made with old metal shelving shelves.
I can post some pictures of my propellers if you wish.
You will find making the propellers quite addictive I have a good selections in various colours ready for future whirligigs.