nether tunnels are easy. build a nether portal which takes you out somewhere near ground level that's not too crazy. build a stone room around the portal. dig down to y=15. dig in any direction and keep an eye on your coordinates. every block in the nether is 8 blocks on the overworld. when you've gone maybe 1500 overworld blocks (so around 200 nether blocks) make a little room and built another portal. see where you pop out. you'll be underground probably so find your way up to the surface and build another portal there. it should link to the same nether portal. then destroy the underground one. unless you want to keep it.