I use just water. I paint it on the back of the cutouts with a paint brush. I haven't ever had a problem doing it that way. I know some people use piping gel and other things, but I just use water. HTH
thank you all for your input. i think i figured out my problem. i was chilling my cake before i put the fondant circles on before, and they kept sliding off. this time i didn't chill, and just used a little bit of water and it worked perfect.