The insufficient information provided to the pilot by the mechanic that installed the auxiliary fuel tank for his failure to install a placard to indicate that both fuel on/off valves should not be on at the same time resulting in a total loss of engine power due to fuel starvation. Also, the unsuitable terrain encountered by the pilot during the forced landing. Findings in the investigation was the installation by the mechanic of FAA check valves into the airplane purchased at a yacht company, and modification of the airplane by the mechanic without FAA approval.