When you are stenciling what is the best way to keep it from running?

Spray the underside of the stencil with temporary adhesive. Just a light coat should work. Then the stencil should lie flatter and closer to the surface you are stenciling. Make sure you are using a flat-bottomed brush, and have it almost dry when you are tamping. Then there will be less paint or ink to run. Best wishes ☺️

Use soray adhesive for stencils. Good luck with your project.

I make sure to blot the brush or sponge good before tapping the stencil. You don't want it wet, you want it dryer than you would if you were painting a wall. Just tap the dryer paint with the brush in the stencil, do not spray or "paint" it in. Also, stencils can be purchased that are made to stick to the object for you to do the stencil painting. This also helps to keep from too much paint under the stencil. Remember to tap and not brush, make your brush almost too dry to leave any paint, and try to purchase the "stick on" for the job. Hope this helps.

https://www.craftsy.com/art/article/how-to-paint-with-stencils/