you really really wont need it. go up the main llanberis path. after a steep steppy section you will go under the railway and the trail goes up steeply to your right. once this levels out a bit, just keep a look out for a trail on your right that crosses the rails. thats the rangers path. once the trail levels out at the bottom you will go along the flat for a bit. it starts to slope downhill slightly and a few waterbars appear; dont let it go, keep an eye out for an almost invisible marker post pointing up the grass bank on your right. if you get to a series of hairpin bends you have gone a couple of hundred yards too far. push up the grass bank to the top of telegraph alley, head back to llanberis. job done!
oh, and enjoy! still my favourite uk trail by far