I'm sure different people will have different technniques but this is what I do.
First I find the top edge of the pubic bone and put the end of the tape measure on it, holding it in place with my right hand. With my left hand I then smooth the tape measure up, over the belly, running my hand over the uterus until it starts to dip down over the fundus. Then I feel a bit more firmly to identify where the fundus actually is and, once I have a clear picture I position the tape measure and press down with the edge of my left hand so the tape is sitting on top of the fundus not just on top of the skin IYKWIM. How firmly to press depends on how much tissue there is between suface and fundus. I guess I press until I can feel the slight resistance of the uterus.
It probably doesn't matter too much though as trends are more important than individual figures so, if you use the same technique all the time, then you will be able to identify growth.