This achievement is fairly straight forward. You must jump from the bridge leading from Silverwood to Sanctum. Jump from the bridge into the water from the Silverwood side. There is a ramp leading back up to the bridge on the south side of the Silverwood bank. Go back to the bridge and walk across until you are in Sanctum. Jump off the side of the bridge into the water again.
You get the title "Base Jumper" for completing this achievement