Bother - netwolf hit enter first. Here's a rundown of the method. It won't work on the Tropic of Capricorn on the December Solstice, or on the Tropic of Cancer on the June Solstice.
Get a good time reference - accurately set watch, GPS, radio, NTP synced computer clock. You don't have to be dead right, but better is better.
Find the time of local solar noon - eg
http://www.ga.gov.au/geodesy/astro/sunrise.jsp and select "Sun Transit Time"
Hang the plumb bob where you want the centre of your mount to be.
Wait for solar noon and mark where the bob touches the ground and as far along the string shadow as is practical.
Extend the two marks to form the north-south line - one tripod leg will sit on that. Mark an east-west line (builders square, 3-4-5 triangle, etc) where the other two legs will be.
Make your final markings however you like - paint, impact drill, etc.