You shouldn't.
There's 2 types of fans. DC have a 3pin connector and are controlled by voltage. That ranges from @ 5v-12v on decent fans, but some Olde junky fans will be 7v-12v. The motherboard changes the voltage to increase/decrease speeds, 5v is slow, 12v is max. Not something you'd control. It's done according to the fan curve settings in bios, according to temp sensors on the board.
The other type is pwm, 4pin, which are 12v always, but the pulse width modulated signal turns the fan on/off. It's always trying to get to max speed, but cannot unless the bios allows max speed from the temp sensor.
Just mount the fans and plug them in. Motherboard will do the rest. If they remain at 100% always, might nerd to go into bios and change the mode from pwm/auto to voltage.