Step 4: Using a Zener Diode
For unidirectional shifting from 5V to 3.3V, you don't get a much easier and straightforward (as well as cheap) solution as using a zener diode. Find a zener diode that matches your voltage requirements, two common ones are 1N4733A (5.1V) and 1N5226 (3.3V). You'll want to match your wattage; these are 0.5W diodes.
By attaching a load resistor inline before the zener diode, and taking the zener to ground, you can ensure that the voltage that gets past the zener is at or slightly below the zener voltage (Vz) of the zener diode. In the example picture below, you can see that inputting a 5V signal outputs a 3.18V signal.
In the second example, I'm "regulating" 5.1V after stepping down from 10V. This isn't to be confused with a constant-current/voltage regulation. This is best used as a signal with little to no load.