I use a mask/template and then airbrush the blur on to a clear plastic disc. I tried freehand masking but the edges of the blur did not look sharp enough.
So after buying a circle cutter I set out to make the mask/template.
Using thin card I marked out the blade postions and a nice width for the segments of the prop blur. (This is a mask for a five bladed prop)
On a sheet of paper, mark out the blade postions, then tack the clear disc over the top.
Now the airbrushing, I use very very thinned black acrylic (I've used Gunze Aqaeous and Tamiya Semi Gloss Black) so you can slowly build the density of the blur up slowly... (The prop blur here is a three bladed, but its the same kind of mask)
Once satisfied I normally give the blur disc a coat of Klear/Future, as the acrylic is rather delicate.
I have tried a yellow tip on the blur, by using a circle cut out as a mask over the blur, but I've not perfected that method yet.
The template/mask is the trick, and very thinned black paint, slowly building the opacity up. Once you made a mask/template its easy to do a few blurs.
Here is the result..