This will be a short article with some tips and suggestions. The base rock I made was my first attempt. You will find that this is a fun project to do with a group of people, especially your children if you have them.
I used the same ingredients GARF uses for the aragocrete:
Time to make some reefrock!!! First we need to set up the mold container. Pour enough aragonite sand into one of the rubbermaids (or as many rubbermaids as you can talk people into helping you) to a depth of at least 4 inches. Moisten the sand, not so that it is wet, but so that it is easy to mold down into and not have the sand spill back into the mold you are making. Mix up the Aragocrete so that it will clump in your hands but make sure that it is wet enough so that the cement will set. You want the cement mixture to be able to stand up a few inches without falling down. When you are ready simply dig into the mold container about 2 inches deep and make a shape about 12" by 6", use your imagination, you might even draw out the shape of the rocks that you want to make and work from the drawing. Put a thin layer of the Puka shell into the mold for some texture on what eventually will be the top of the rock. Fill the shape you have made with the cement mixture, being careful not to leave any gaps that will weaken the structure. The sticks are for poking holes through the pieces and for forming the rocks around to make the rocks more interesting. Make sure you pull the sticks out and fill the desired holes with dry sand (you can use a masonry bits of varying sizes, when the rock is fully cured (12 weeks), to drill holes and give the rocks a more natural look). The first few pieces may not please you but it is a learning curve, and you will soon be making arches and other structures. When you are satisified with the shape, cover the piece you have made with some of the moistened aragonite sand and let the container sit covered for 48 hours. You can then pull the piece out, brush off the loose material and place it in another empty container (if you have the space a kiddy swimming pool is ideal for this process). Cover the piece(s) with fresh water. They will now need to sit for about 12 weeks to get rid of the alkalinity in the cement. Change the water frequently (at least once a week if not more often).
There is a lot of information on the web about this process and using vinegar and hot water to speed up the process. I personnaly was building the tank at the time and also gathering the materials and knowhow to construct all of the other projects that were on my list (skimmer, overflow, sump/fuge etc.) so i had time to let the rock cure on its own as I was not in a real hurry, after all I was trying to do as complete a DIY reef tank as I could pull off.
There are some good pages on shaping the rocks into forms such as tables, for the base structure of the reef formation, arches and caves on the GARF site.