The first thing you've got to get right is the alignment. Check that by measuring the distance from swingarm to wheel on both sides. You can also put a straight edge against the rear tyre like a scaffold tube etc, then check that it's equi-distant from the front tyre front and rear.
Once you've got the wheel straight it's then a case of finding the tight spot on the chain. Do that by rotating the wheel slowly and pushing the bottom run of the chain up and down with your finger. The tight spot is the point you'll adjust the tension at from now on, so make a mark on the chain sidewall, ideally at 3 o'clock around the rear sprocket to find it easily each time.
You tension the chain there because if you used a looser point the chain would be like a bowstring at the tight spot and would wear really rapidly. All you've got to do then to get the tension right is to make sure you adjust each side by exactly the same amount and double check it with your ruler as you go.
Less haste, more speed and you should be fine.