I've only stenciled on fondant once, so I'm no means an expert.
I can tell you that you need to practice on parchment before doing the cake. You can make different consistencies to see which works better. I used a med. consistency RI. It needs to be easy to spread, but not too thin that it will not hold shape. I've read that you can use painters tape to secure the stencil to a fondant cake, but I was too chicken that it would mess up my fondant. I had someone else hold the stencil while I applied the RI. You want to apply the RI evenly on the stencil, so the design will be the same thickness. I also found that it was easier to apply the RI in one direction. After carefully removing the stencil, allow it to dry before doing the next one.
You can also use buttercream, but the RI dries very hard to the touch. I'm a klutz, so I need all the help I can get.
I also tried a piping gel stencil mixed with luster dust on BC which was a little more difficult that using RI, but was beautiful and shiny.