Most of the times you can use just your fingers to hold the nut in place while you tighten it with an allen wrench and it will tighten fine. But sometimes you can't get one bolt fully tightened because it would spin round and round. There's a special tool that Park makes that has three prongs that you insert to the end nut so it doesn't spin. Maybe your chainring is loose because you can't get one bolt all the way tight???

And use a Star pattern when you tighten it. Don't tighten it going around. I normally tighten them all few threads at a time in a star pattern, get it all seated and then go to town again in the star pattern to fully tighten them.