I only have a few mantids eating fruit flies right now so I haven't made feeder holes or any tools to help, but it hasn't been too bad. When my Chinese mantids were small, they were in fairly large enclosures so it wasn't too hard to just tap some fruit flies in straight from the culture without worrying about escapees.
I now have some small ghosts living in deli cups and tapping the fruit flies from one 32oz cup to another seems riskier because some could miss the cup. What I'm doing right now is taking the culture outside (so escapees don't matter, just in case), and using a funnel to tap flies from the culture into small deli cups. If any get out, I coax them into the small deli cups with a spoon, which is easiest when they get to the edge of the table and I can just push them right in over the edge.
It's a lot easier to just dump all the contents of the small deli cup into the larger mantis deli cup without any spilling out; in fact, I haven't had any escape during that step yet.
Of course, I'm sure my method could get tedious if you have larger numbers of babies to feed, in which case the above suggestions are probably best. Either way, good luck with it!