2 ways, either glue it to a frag plug and try out different locations, when you found the best spot, simply snap the coral off of the plug at the base and then glue to coral to the rock. A lot of people do that. Another way is to gently wedge the coral between 2 rocks, or in a hole in a rock were it will stay put for a day while you observe it to see if it will work there. 2 main things your looking for with placement is light intensity.. is it too much/too little and flow, same thing. Some corals like strong flow, some will die in strong flow.. some like really intense light, others will bleach out and die if the light it too much. If you observe the coral on a daily basis when first introduced you should be able to spot any problems as soon as they arise and move the coral accordingly.. if it is turning white/colors fading, usually too much light.. if colors look good but polyps don't come out, probably too much flow.. This is of course assuming all your water parameters are in check like pH/Nitrates/Alk/Calcium.