There is a bit of technique involved in using epoxy to mount coral to the rockwork in your reef. There are a number of techniques that would work from what I've read here in RC, but here's what I do and it works:
1. first, determine from the size of the coral rock that you want to mount, how much of the epoxy you'll need to make a good connection and mount...This may take a few times so that you get a sense of the amount.
2. Knead the epoxy so that the two colors are thoroughly mixed and the "putty" is now one color
3. Put some Crazy Glue Gel (or other brand but it should be Gel) on the back of the rock you want to mount, dip it in the saltwater, then put another dab on top of that...put it onto the "putty"
4. Put some Gel on the other side of the "putty" and then dip it into the saltwater, and then put another dab of Gel on that
5. Firmly place that side of the "putty" on the rockwork in the tank where you want the coral to be mounted.
6. Depending on the angle of the mount and the size/weight of the coral rock that you're mounting, you might want to hold the whole thing in place for a while so that it sets right and makes a good connection and starts to harden so that you can rely on the mount that you've made.