If you buy the Shearwell applicator, it comes with a very good explanatory leaflet. This is reproduced on their website
here but the line drawings are not so clear on my screen.
They also have some top tips on their website
here; there's a link to a video showing Richard Webber tagging a lamb and a ewe according to Defra guidance too.
Top/front is the leading edge of the sheep's ear - the edge that will come to you first as the sheep approaches you. Rear/bottom is the trailing edge, the edge that leaves you second as the sheep moves away from you.
In the video, Richard tags his lamb in the upper/front edge - interestingly, he says the muscle is thicker there, implying that the tag will hold better on that edge. When he does the ewe he doesn't use the strip tag, he uses a button-and-flap tag, saying that the ewe's ear is very thick and this type will work better. I was also interested to see that he has the button inside and the flap on the back of the ear; he says that this reduces catching on wire fences and shows why.