How do I do uncertainties properly?

All uncertainties show you is the range your answer will fall in. First off, when you do the experiment, any analogue scales will have an uncertainty of half their lowest unit and digital scales will be their lowest unit. This is your absolute uncertainty. When adding and subtracting, sum up the absolute uncertainties. You get relative uncertainty by dividing your uncertainty by the value. When multiplying or dividing values, add up the relative uncertainties - don't multiply them! Don't forget to convert all your uncertainties to the same - absolute or relative.