Generally it's not advised to jump start. If you must then it's important to follow a few simple rules (but I don't know what they are....)
Some people suggest turning on a few electrical loads on the car doing the jump starting to lower the battery voltage as this drops the voltage difference between the 2 batteries whilst connecting the jump start cables.
Hook up the positive of the flat battery to the positive of the good battery, then the negative of the good battery (and this is the important bit) to a clean metal earth point on the engine of the car that is to be jump started (not the battery negative terminal/post). Start the car doing the jumps starting.
At this stage depending on how flat the battery is I tend to just leave the good car running with it connected up to get a bit of a charge going into the flat battery, then after little while attempt to start the car.
IF all else fails, read the owners manual as it will have the manufacturers guidelines on jump starting with said vehicle.