Where exactly is the fuel shut off button located on a 1990 Ford F150 xlt Lariat the PU is standard?

If you're talking about the inertia switch, in my 1990 F150 XLT Lariat, it's located on the firewall, driver's side, beside the main wiring harness pass-thru, directly below the brake pedal linkage. It's black, with a red indicator on top. I've used it before to drop out the fuel system so I could disassemble the fuel rail.