I put them in the middle mate. The tunnel can look like anything you want, I have some tunnels I made out of planks that are the exact dimensions for a trap to fit in set and fired that are about 18"-2ft in length which work well round sheds etc. In the woods etc you can use the same things but disguise them or make a tunnel out of logs or stones etc or just use a natural burrow but its more awkward. Some of the lads on here use wire tunnels on rat runs that i've seen.
I always set the trap flush with the soil level and sprinkle some fine soil over the top of it (while the safety is on) alth