obvious that it sounds you need to have 2 two way switches (the ones with 3 terminals on the back, and a new switch base, but they usually come with pullcord switches)
You also need enough 3 core AND earth (red, yellow,blue and earth)cable to go from one switch to the other.
The existing two wires go into L1 & L2 (doesnt matter which way)
then you do this (again it doesnt matter which colours but if you stick to this it WILL work)
from the existing switch postion (but new 2 way switch)put the red in L1, blue in L2 (along with each origonal wire) the yellow goes into C (common)
At the other switch (new position, new switch)since you only have the 3 wires (and earth)the red goes in L1, the blue in L2 the yellow in C (common)
all earth wires (three in all, one existing, two new, one at each end) are connected to the earth terminals in the switch base NOT the switch itself ( so thats two in the origonal position)
But you will turn the mains off first and do this in daylight wont you!
Electricity KILLS, it has no prejudices, so as mentioned turn the mains off first
it sounds simple because it is, but do follow the instructions in Reverse order (turn mains off first)
U.K