I use peat filtration in my blackwater tanks. I wouldn't recommend it as a substrate unless you really know what you're doing and you are very serious about substrate maintenance.
You can buy special aquarium peat from Hagen and Sera....it's in compacted little balls so it doesn't make a mess of things - but I have found it's not very cost effective and that you can easily use old nylons (or just swipe some of those things they have in shoe stores for trying on shoes). The trick is to fill them with the peat and then hold them under running water, squeezing them every few seconds until as much of the powdery, messy stuff is gone.
The best way to determine how much you need to use and how often you need to replace it is to know your original water parameters (hardness, pH, etc) and then start with just maybe about a half a cup per gallon. Spend the next two weeks testing every other day to see what changes. If it's too much, lessen the peat. (Truth is it would take a lot of peat to make any serious impact on very hard, alkaline water.)
What you should be looking for is just a very slight, gradual decrease in hardness and pH over the first week or so. Almost unnoticable really. Spend the next week seeing if the effect continues, stabilizes, or reverses. Adjust according to your wishes.
Again I just want to remind you that you shouldn't be using peat with any expectation of turning your hard, alkaline tapwater into instant blackwater conditions. Not only would it take immense amounts of peat and a really long time to do this, it would also make water changes a shaky affair because of the drastic difference there would be between your tank water and your source water.
So the main reason why I use peat filtration, driftwood, etc is to create blackwater conditions. It's more of a tannin / water conditioning thing. Blackwater fish are just happier when there is that essence of their native habitats. Even before there is the slightest change in my pH or hardness I see my fish coloring up, acting more lively and just generally being super happy in the amber colored beauty of peat filtered water.
So to make a long story a little shorter, use peat more for your fish's benefit and for aesthetics rather than for serious chemistry applications. As long as your water's chemistry is constant, most fish will be perfectly happy. Actual pH and hardness aren't as important as consistency.