How can I convert a single light/fan switch to separate switches?
Assuming that you currently have 2 wires (plus ground) going from switch to fan, you will need to do the following:


Add a 3rd wire. If you have individual wires then you just run another wire (make it a different color from the existing wires) through the conduit from the switch to the fan. But it is quite likely you have a /2 cable - black (switched hot) + white (neutral) + ground in one cable. In that case, replace the cable with a /3 cable - black (switched hot 1), read (switched hot 2), white (neutral), ground.
Remove the existing switch. You may need a bigger box to properly handle 2 switches - that is based on box fill rules, which I conceptually understand but I am not an expert on that.
Attach the black wire (hot) from the panel to either the common hot connection on the switch (some dual switches include a common hot connector, which makes things easier for situations such as this one), or wire nut (pigtail) the black wire with two short black wires and attach each of those to the hot of a switch.
Connect each of the two switched hot wires (typically one black, one red) to the switched hot screw on a switch.
Use a wire nut to connect the panel white neutral to the neutral wire going to the fan/light.
In the fan/light junction box, connect each switched hot to one of the hot wires on the fan/light. Note that the colors may vary - typically black, red or blue.
In the fan/light junction box, attach the white wire (neutral) from the switch box (which is connected to the white from the panel) to either the common neutral connection on the fan/light (some fan/light combinations common neutral connector, which makes things easier for situations such as this one), or wire nut (pigtail) the white wire to the separate white (neutral) wires for the fan and light.
In both switch box and fan/light box, all grounds should be connected (wire nut) together.


Note that this is based on panel/power -> switch box -> fan/light. If your configuration is panel/power -> fan/light -> switch box then the setup will be a little different.