There are many different ways of mixing concrete, depending on its application. For foundation work you want a very hard concrete, one that is able to withstand as much compression as possible. For this you want a very precise mix of concrete, sand, small aggregate and water. To ensure you have a proper mixture you'll want to use a metal cone that is specially designed for a cement "slump test." It is also a good idea to measure your ingredients in a bucket rather than measure by shovelful, as a bucket will give you a much more accurate measurement.
Purchase all of your ingredients: Portland cement (generally sold in 25 pound bags), sand, small aggregate, mixing tub or cement mixer (purchased or rented).
Pour your cement into your measuring bucket. Note how full the bucket is. This will be your benchmark for further measurements. Pour the dry cement into your mixing tub or into your concrete mixer.
Add precisely twice as much sand as dry cement, measured with your bucket.
Add four times as much aggregate as cement as measured by your bucket. Aggregate cannot be more than 3/4-inch in diameter.
Use your bucket to measure an amount of water equal to 55% of the amount of cement. In other words, add just over half as much water as you did cement. Mix thoroughly. Concrete should be about as thick as pancake mix.
Place the "slump cone" on a flat surface and fill with concrete. Ram a small stick through the top of the cone and into the concrete to remove any air bubbles. Carefully lift off the metal cone. Approximately 1/3 of the cone should "slump." If more than 1/3 of the concrete cone sags add more concrete to your mix. If less than 1/3 sags then add small amounts of water to your mix until you get a proper "slump."
Wet the ground before pouring your concrete. One hour after pouring cover your concrete with burlap or cloth tarps and wet thoroughly. Keep the tarps wet for 14 days. Remove tarps after 2 weeks but continue spraying foundation cement with water for another 14 days. Do not build on the foundation until it has cured for 28 days.