Fill up your bottle with a known amount of warm or hot water. The hot water just helps the crystals dissolve faster. My bottle comfortably holds 8oz of water. Also place a sheet of paper beneath the bottle to catch any stray dichromate crystals that may fall.
Use Caution!
Dichromates are chemicals and are dangerous to your health. Consider them to be poison at all times. However if proper precautions are taken, there is little to worry about. Refer to the Material Safety Data Sheet for more information.
An Ammonium Dichromate solution stored in a bottle ready for mixing
Ammonium dichromate crystals resemble grains of salt, only they have a striking orange color and are poisonous. I use an 8oz travel shampoo bottle seen to the right to hold my dichromate solution. I marked the side in fluid ounces using a sharpie and labeled the bottle as poison so hopefully no one will not try to drink it. It unfortunately resembles orange Kool-ade and might be attractive to children, so if there are children around, keep the bottle of solution out of sight and reach.
Dichromate crystals dissolve readily in water. Here is a handy calculator I built to figure out how much ammonium dichromate or water you should use for a 1:10 solution. The conversions take into account ammonium dichromate's density which is 2.15g/ml.
I often see that you should mix ammonium dichromate to its saturation point, where the water can no longer dissolve the dichromate crystals. Ammonium Dichromate solubility is 36.4g /100g water at 68F according to the MSDS. I have never mixed my dichromate solution to its saturation point so I cannot comment other than it seems to be a waste of dichromate and thus a waste of your money.
Instead, I learned to mix ammonium dichromate at a more diluted ratio of 1:10 with water, and have always done so experiencing no problems with its effectiveness. A stronger or weaker solution may have a noticeable effect on exposure times, but I have never tested for this.
Mixing Instructions
Measure the amount of dichromate crystals needed from the calculator above. For 8 oz of water I need about 10 ml of crystals, or 2 & 1/8 tsp. I use an old plastic film canister with ml markings on the side.
Using a funnel (or you can roll a sheet of paper or index card into a cone as I do), carefully pour the crystals into the bottle of water. Do it slowly and in a manner that will not create dust. Dichromate dust can get into your lungs and on your skin if you are not cautious enough.
Take the sheet of paper beneath the bottle and gently fold it in half and funnel any stray crystals into the bottle as well.
Finally, close the bottle tight and shake it gently a few times letting the crystals dissolve into the water. Once it's dissolved in a few seconds, it is ready to use!
Storage
Remember to keep it in a safe location because it is poison. I keep mine with my other printing materials on a shelf. Unlike other photographic chemicals, an ammonium dichromate solution can be stored out in the light with no problem. I'm not sure how long it will last sitting somewhere, but I once had a bottle of solution sitting around for over a year and nothing changed about its quality or the way it worked. I imagine the worst that might happen is the water in the solution would evaporate and leave behind dichromate crystals. Just add water!