Lift the non-power spider or chainring assembly off the crank and replace it with the power meter version. Install the eight new Torq T20 bolts supplied with the power meter and torque them to 4 Nm. If you have a Force eTap AXS crankset, now install the chainrings and torque the four Torx T30 fixing bolts to 12 Nm.
Reinstall your crankset and torque the 8mm hex fixing bolt to 54 Nm. Just be sure to place the chain on the small chainring before doing this.
The last thing to do is to install the power meter battery and then follow the instructions for your computer head unit to pair your power meter to the device.