I was asked on the Roden Lozenge Decals thread if I would like to show everyone how I have developed a method for cutting stensils for and spraying roundles and other markings.
Now that my wife and I have settled in in our new home, I'll make a start.
Here's a shot of the tools that I believe are necessary as a minimum to create your own stensils.

The compass you see here was inherited from my Grandfather, who was an industrial artist. I don't know if these are available in any shop in this form anymore. Scrounge the second hand shops... You might be lucky.
And here a shot of the two different materials I use for masking and stensils.

The Leukoflex is an adhesive film that I picked up at the local pharmacy. The other tape is just 40mm Tamiya brand. The Leukoflex is cheaper, and I find it easier to use as it has no fibres that might create rough edges.