Just make sure the ears are fairly clean (but don't wash immediately beforehand, otherwise it might affect the ink sticking).
Standard practice is inside the right and outside the left - i.e. as you look at the pig, the right ear has the tattoo on the inside, but you have to go behind it to view the tattoo on the back of the left ear.
I was taught to cover the area to be tattooed with lots of ink before using the pliers and then, after crunching them into the ear, apply more ink and work it into the puncture holes with a toothbrush.
Tattooing is a horrible process, and I would recommend wearing ear plugs! Weaner screams can be louder than those of a jet engine, apparently. Makes it a lot easier if you're own ear drums aren't suffering.
Another thing - should have mentioned earlier - MAKE SURE TO KEEP THE SOW OUT OF EARSHOT! Lock her in another barn or paddock for the duration. If she gets out, she could kill you.
Good luck
Liz