Take an old paint brush and drill down into the ferrulen or bristle sleeve.
Cut a notch out of the side and sand all edges smooth.
Take a pin for 54mm scale, a piece of brass wire for bigger scales and after rounding off the point,
glue it into the paintbrush sleeve making sure there is a space all round betwerrn the pin and the metal sleeve.
It also needs to stick out slightly.
Press it into the putty over and over to make a line of ring impressions.
At the end of the row, roll the tool in your fingers and do another row.
The notch in the side helps create overlaps and is quite realistic.
Practise on some blu-tac or plasticine first and you make fewer mistakes.
You can make different sizes of tool for different scales
Hope this helps
Paul