I use organic peat from nurseries. I moist it and cook it in the oven for 1hr at 200'C, and then let it out to dry for a day. I then put it in a zip bag with very fine mesh and place it in the filter or sump.
I use approximately 1gr of peat per liter (which roughly corresponds to 2.5ml of store-bought peat, in granules). The amount you use is dependant on how much you want to drop your water hardness, and how fast.
The problem with peat is that it's very active in the beginning (so, big decrease in GH, KH and pH) but it's activity slowly deteriorates. After about 4 weeks it needs changing.
To avoid big spikes in pH drop (which is dangerous for the fish) I stick to the above mentioned peat:water volume ratio, and separate the peat in smaller socks, which I sequentially change every 3 weeks. One trick might be to use colored stockings so on week 3 you change the blue, week 4 the red and week 5 the black one, week 6 the blue, week 7 the red and so forth.
For your reference my tap water is pH 7.5, GH 14, KH 10. With peat the pH is now 6-6.5, GH 9, KH 2 (measured with sera and API kits).
Hope this helps, looking forward to reading what other people use.