In the course of learning the "best practices" way of doing this I spoke to peat producers, professional plant growers, a couple botanists, sales people and several enthusiastic and helpful hobby growers. Everyone had an "I killed my plants story." and everyone had something interesting to add.
Step 1: The components and "tools"
Sand- See below
A large mixing bowl
A measuring cup of some sort- I used the bottom half of a glass soda bottle.
Peat moss, specifically sphagnum peat. Not to be confused with green moss, long fibered peat or carpet moss, all of which can be found in bags calling them "peat moss". There are differences and they usually have negative effects. Almost all are not immediately evident and will only show up after weeks or months of seemingly healthy growth though some chemicals that are found in certain products may kill your plants very quickly.
I used Uni-gro premium organic peat. It's distributed through L&L Nursery Products here in California. It doesn't state the source on the package so I called them. It's Canadian sphagnum peat that comes to them from a company in Alaska and is then packaged by L&L for resale. This is a good one for US growers. No chems, fertilzers, wetting agents, just peat moss. It was just a few dollars for a 4 quart bag.
The other component I used was a very clean chemically inert black cover sand from Mosser Lee. Again, no chems, additives, fertilizers or anything else. It's cleaner than the washed sand recommended by more sources than any other component and it's non-leaching so you won't need to worry about mineral content over time.
Distilled water. Yes, you need this to make the soil. Rain water will do but it is so easily contaminated you'll either need to control the harvesting process carefully or just go ahead and buy distilled water. This is not the same as bottled water and chlorine isn't the big offender, so setting a bucket of water out for 24 hours the way you might do for fish will not make it safe for your plants. Many bottled waters have salt added for taste. If you must, you could buy low sodium drinking water but it's a poor risk with distilled water easy to obtain. Soft water is not acceptable for the same reasons. While these things might not kill your plants right away, some will and all they will over time. Again, the salt in Bottled water added for flavor is negligible as a food additive but will accumulate over time and harm your plants.
Notes on other materials:
Perlite or pulverized lava rock may be used in place of sand if it is additive free. Just make sure it doesn't have a fertilizer component like miracle grow and maybe call the producer and bug them about salt content as well.
Coconut husk will work but requires several rounds of soaking to force out the small amounts of salt that are present in the coir. If you really want to use coconut fiber and have an extra month to process it there is information out there. Using it as is, out of the package, will likely kill your plants after a few months of watering when the salt finally starts to effect the roots.
I did not do a soil salinity test on any of the materials but did do a pH test on the final batch for the plants and based on direct information from the producers of the products was assured that there were no salt or chemical additives.