Like... an aftermarket one? What do you want to know? Run sender wires through the firewall, find a good power/ground for the gauge itself... done. Or if its mechanical, punch a hole in the firewall and install the supplied rubber grommet for the mercury line and then run the power/ground as normal. Its really pretty easy.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.