Now that you have all the ingredients, it's time to mix.
The proportions are somewhat important, somewhat unimportant. If you are using silica fume (adds a lot of strength, but may be unnecessary) use 1/7 to 1/10 weight of portland cement. Use about 30% to 40% (weight) water, and 1 to 2.8 oz superplasticizer per pound of cementous material (or whatever your manufacturer recommends). More of this stuff is not better. After about 3 to 4 ounces per pound you will start to decrease the strength of your concrete (again, I don't know if this will cause your holds to be unusable or not. I don't have enough data yet). And add some sand. I use 1 to 2 times the weight of sand to concrete. Just eyeball it and experiment. I recommend that you record your mixes in a notebook so you can see what works well and what doesn't.
A mix that I've found that works well for me is 1 pound of portland cement, about 1.6-2oz silica fume (about 10%), 2 pounds of sand, about 7 ounces of water (about 40% weight of silica and cement), and about 5cc of plasticizer ( 0.15 by weight of cement and silica.)
Add the plasticizer last.
It's very difficult to measure everything out if you don't have a balance, so you'll be eyeballing everything, and this is OK. But I reccomend that you get a small kitchen scale to weigh things out. If you are just experimenting and don't want to shell out the $15 for this, just eyeball it, as that will be sufficient. If you decide that you want to make more of these though, a scale is a good investment.
Mix well. It will be very hard to mix with this little water. I find that it is easier to mix if I mix all the dry ingredients first, and then add the water, and finally the plasticizer. It should get pretty liquidy and easier to mix now. If it's still not too easy to pour, add a little more plasticizer or water until it's easy enough to pour or scoop out with a medium sized putty knife.
Pour your molds. You can use vaseline, silicone, or nothing as a release agent. I usually spray a little silicone or put some petroleum jelly on the part of the mold that forms the hole for the bolt and the washer if it's made of wood (sometimes the silicone plugs will rip, and you can extend the life of your molds if you replace the ripped ones with wooden dowels.)
After a few hours (varies with humidity, temperature, and use of accelerator) your concrete will be hardened. De-mold it, wrap it in a water soaked (but otherwise clean) rag, and put it in a plastic grocery bag to moist cure for at least one week. Alternatively, you can submerge the entire hold in a large pail of water. NOTE: after some discussions with someone who knows what he's saying, this last method is not advisable. Buy some burlap and wrap your holds in this. Submerging your holds in water can leach your calcium out weakening your concrete. The silica will help to prevent this, but it's still inadvisable.
After a week, you're ready to use it. The longer concrete cures (see section on curing in Technical Information), the stronger it gets. It also gets stronger when it's cured in a wet/humid environment. I usually wet the sand that I have surrounding my molds and put a lid on the container to trap the moisture just after I've poured them. Also, you can turn your molds over onto a scrap piece of plywood after filling them so that you'll have a perfectly flat back.
There are NO fumes or smell from this procedure, unlike with plastic, so you can do it in your basement or wherever.
You can experiment by adding color. Colors are available usually right next to the concrete at HomeDepot. The colors are usually iron, cobalt, or copper based. Any other pigments will quickly fade due to the chemistry of the concrete.