See page 1 of my tank thread (more info on the old 50 gallon reef link).
I forgot the ratio I did, but I just mixed portland with beach sand and salt then molded it to shape. Misted it everyday with freshwater then after more than a week, dumped it in a large plastic container full of water. Changed the water almost everyday, or if I felt like doing it until the PH was fixed.
During curing (the setting of mixed concrete), all phosphates will leach out from the concrete because of its high PH. That's perfectly normal (and good), because as soon as the PH stabilizes, you will have clean, phosphate-free rocks.
Other things to consider:
Use portland cement, you may use white portland as well but it will be less sturdy.
Don't let the rocks dry out prematurely, it will form cracks and will not crystallize properly. Don't let it dry out in the sun either until fully cured. If the concrete dries too quickly (is not cured) it will not be stable.
Salt is optional. It is just used to make the DIY rocks more lightweight and porous. Some use spaghetti or other organic material, but I don't like the idea of putting more crap in my tank.
Do not use vinegar to accelerate curing. If you must, use it on the last week of curing, so the rocks are already set and sturdy.
Have fun