Brute force way, bypasses everything:

Mount a toggle switch somewhere.
Connect one terminal to +12v through a fuse, other terminal ties in to the red wire powering the fan.
A blue "scotchlock" tap conector- the kind you squeeze with a pliers- is a simple way to connect your new wire to the existing one without cutting it up.
As LongArm mentioned, this leaves your existing fan controls in place, just adds manual control on top of them.
IIRC that fan circuit is rated for 30 amps, so assuming 15' or less wire from switch to fan, use 8AWG wire at minimum, a 30 amp fuse and inline holder and a switch rated for at least 30 amps.

There are other ways to do it, including grounding the coil on the fan relay or shorting the radiator coolant temp switch, but those still rely on the relay, fuses and ignition switch circuitry to operate.