OK, GTAA, many of you have been asking me about my personal experiences with my DIY live rock. There are many online guides and Youtube videos out there, which are a very good help and can show you how to do it; I'd suggest you check some of them out, as each of the techniques give a unique texture and style to the rock. I'll share what I did...
Wanna save some cash? The whole DIY live rock craze is simply making your own custom shaped rocks that serve the purpose of reef-harvested live rock without the ecological impacts. Though it takes longer to cure than natural LR, you can fill your tank for cheap $$$ and and do a real custom aquascape and rock walls.
Basically we're taking non-silica based aggregate and binding it together with Portland cement. But we want it to be porous, right? That's why we will also combine salt into our mix, as when cured, the salt will dissolve and leave empty pockets behind...
Please also bear in mind that any concrete product will take 30 days to cure, and cannot be used in a live tank until it had fully cured, as it will leech into the water and alter the pH quite significantly.
Step 1: Pick your aggregate. Use crushed coral or crushed oyster shell. A 50 lb bag gave me more than enough to do my 35 Gallon tank. Don't use brick sand or any other decorative sand unless you know it is not silica- based. Silica should be avoided in a SW aquarium in order to control diatomic algae growth.
I got my crushed oyster shell from my local CO-OP store for about $15.00 They use it to give egg-laying chickens more calcium in their diet.
Step 2: Portland cement. This will permanently bond the rock together. Use regular or white if you can get it, but it doesn't really matter. Once again, a 50 lb bag is more than enough, as you will only use about a third of the bag. See if you can bum some off of a bricklayer buddy of yours

Otherwise it's about 10-20 bucks or so a bag depending on size.
Step 3: Pick your salt. Use any salt you want; bigger grains is better than smaller. I used a 10 kg bag of road salt life this, and I still had some left over; cost was about $6.00. Lowes also sells a cheap kind of water softener salt that has large grains.
Step 4: Mix it together! This is the hardest part. If you can call it that. If you get this, you're laughing! Use the ratio of 2.5:1:2 for Aggregate, Portland cement, and salt, respectively.
First mix the aggregate and cement; then add water until your mix is at a thick sticky consistency. You want to be able to form it into shapes and stuff, so don't add too much water or it will just slop all over.
Then add your 2 parts of salt, and stir well, but for no longer than neccesary, as the salt will begin to dissolve if you stir it too much.
Step 5: Make your rocks! This is the fun part!

Ask your kids if you can use their sand box. Or just use it without asking.
Make sure the sand is damp. Dry sand will not work. Make some interesting shaped holes and depressions in the sand, and then plop some of your mix in and shape accordingly. Get your hands dirty and have fun, but wear gloves if you have sensitive skin, because Portland cement is quite acidic.
Step 6: Cure. Wait 48 hours before you take the rocks out of the molds. Sometimes they can be taken out after 24 hours, but you risk breaking them. Submerse them in buckets of water
for the next 30 days, changing the water about once a week or whenever you feel like it.
Once you set up your tank, be prepared that the pH may fluctuate for a couple of weeks; but I did this while my tank cycled, and I had no issues. I did my first water change about a week after my tank finished cycling and the pH has been stable ever since.
Step 7: Seeding You will need to seed your tank with either live sand or live rock from another well-established tank. LR rubble or sludge from somebody's refugium is great for this.
I think that the DIY live rock may be more prone to the regular algae blooms when a new tank is being set up, but I have not solid proof to back up this claim. It has now been in the cycled tank for about 3 months and is starting to show coralline growth.
That's all for now; I'll add more info as questions and answers arise.