How do you connect a bathroom fan and light switch to on switch?

Bet you meant to say "to one switch" right? The correct method is to get a 2 switch combo made for a bath fan/light. Both switches will use the same white wire in the box as the neutral but the box black is hot and needs to be split to the hot sides of the new switches. Then the fan and the light each get their own power from the other side of the switch, the switch never sees or touches the neutral wire.