I like to use a mix of 1/3 peat moss, 1/3 vermiculite and 1/3 manure or compost. When I mix up a big batch of soil for the raised beds, I use a large block of peat (107 litres or 3.8 cubic feet), a large bag of #2 expanded vermiculite (118 litres or 4 cubic feet) and four bags of manure (4×10 kg or 22 lbs). The manure or compost should be a mix of different types: steer, sheep, mushroom, worm castings, or whatever you can get your hands on. The more variety the better.
I use a large tarp spread across my lawn and a rake to mix with. Start with the peat moss, spreading it out and getting rid of all the clumps. If you value your ability to breathe then be sure to keep the peat moss moist while working with it. Next, add the vermiculite again making sure to keep the dust to a minimum. Mix them together and add the manure. The pile is getting fairly large at this point and to get a thorough blend you will need to lift the tarp from side to side to mix the ingredients.
Once the soil has been well mixed it’s ready to use.