Following manufacturers guidelines, I use a mix of 3 parts sand to 1 part cement, with enough water to make a "stiff" mix of mortar. Using this basic formula, (it can be as much as 4 parts sand to 1 part cement), other variations can be tried. For example, I used a latex polymer tile set product as one part of the sand allotment. So it became: 2 parts sand, 1 part polymer, 1 part cement, plus the water. This adds some sticking power to the mortar mix, and makes it easier to control, I believe.