There are definitely various ways to create templates. This is just my way to do it.
This is just an example and a description how I make mine with the help of other artwork.
I scan the original artwork, I'd like to create an template from.
In this example it's Element Lad by Ron Frenz.
2. Change of Colours
First the scanned picture is turned into a grey-scaled picture.
Then back into a RGB- or CMYK-mode.
The grey colours are turned into blue and the brightness is encreased.
Finally this version is printed out.
On the print-out I draw directly the outline of the template with black.
4. Second Scanning
The print-out with the black outline is scanned and the blue shaded pixels are erased.
The different dark grey/black pixels are all coloured to one 100% black.