no need to be very specific , it isn't very hard to find.

First you need to find the museum of history. Enter the museum , then continue straight
forward , you will see two large doors , as well as a very large skull above the doors ,
this is the entrance to underwold , enter it and you will be greeted by ghouls.

If you haven't found the museum of history yet , there is a metro entrance/exit for the
museum , and is also easily found by looking at the metro signs inside the tunnels.