It's tedious, but not difficult and ends up being way cheaper than Promix, ~$30 to make ~10cf of essentially the same thing.My real interest is in wetting peat moss as I would eliminate any pre mixed media like Promix or Fafard.
I do screen the peat moss to expand it and get rid of those 'knots' that never seem to break down or absorb water. I'll dry mix in a wheelbarrow, saving the perlite till last along with the water. Mix everything well and leave a depression in the middle. Pour in about 1/2 of the perlite and wet it down with hot soapy water. Keep adding perlite, water and mixing. I've found that it takes close to 7 gallons of the hot (tap), soapy water for a wheelbarrow full of mix, ~18 gallons.
Water is heavy and that's why it and the perlite are added last. I usually let it sit in the wheelbarrow overnight at least (under cover), and mix again the next day to catch any missed dry spots. I'll usually let it sit till the Santa's beard forms and then into whatever containers to finish cycling All the containers have drainage and no further mixing is done. Over the years I've found that to just be needless extra work. But, if you feel energetic ....
Let it cycle for at least 2 weeks and then, have at it.
HTH