Concrete Mix is a blend of sand and rock. The general rule is to mix 1 part cement to 5 parts concrete mix. Too much water can reduce the strength and durability, as well as cause cracking.
Mechanical mixers are available from equipment rental stores and make mixing easy. Remember: easy is good.
When mixing by hand, form a crater for adding water and mix 2-1/3 to 3 quarts per 50 lbs. cement mix. If its too stiff, add more water a little at a time. Avoid a soupy mix which can cause problems.