You push in the middle, right where your belly button is. If you start there, you can push in and down each time as you work your way down your belly. How much you push in will probably have to do with how much fat or flab (I have the flab, LOL) you have on your belly (the more flab or fat, the more agressive you will want to be to make sure you feel it) You want to really feel, but it shouldn't be uncomfortable or hurt (you don't have to push in so much that it hurts ya know). But don't be afraid to really feel. If you have ever had a midwife or ob palpate your uterus, or baby, you know they can really get in there and feel but it doesn't hurt (unless they are being rough) and it doesn't hurt baby.
If you start up and work your way down, it should be really easy to feel where it gets firm. If you try in the morning with a full bladder, your uterus will be even higher. Once you get the hang of finding it, you can feel it growing, and then will know what you are looking for when you are ready to measure if you want to do that.