1) Load up the mission "A Personal Contract - Mansion Ground Floor".
2) Open the blinds in the house.
3) Go downstairs.
4) Stand by the window.
5) Shoot the shell to the middle left on the second floor.
6) Shoot the shell next to the piano.
7) Look out the window to the lake.
8) Wait a few seconds.
9) You will hear a large moan and then the Loch Ness Monster will rise from the lake.

It does not matter which order you shoot the shells in or where you stand, however you get the best view by the window.

Here is a video tutorial: