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OK, how I always conceive of the schematic of a voltage divider is the schematic on the far left. Your Vin would be the 5V digital signal (NOT VCC, the signal itself) you were trying to divide down to 3.3v (remember it can't do the up conversion, it'll only go "downhill") which appears on the Vout.
I even have the optimal resistor values for you that make sure the currents right, strong enough to drive the logic but weak enough that it doesn't consume any power really.
For 5V to 3.3V signal conversion:
R1 = 1700 Ohms (or a 1.7K resistor)
R2 = 3300 Ohms (or a 3.3K resistor)
Breadboard it out first. Take two resistors of the previously stated values, manually apply a continuous +5VDC to the Vin, tap it between the two resistors (don't forget the ground connection its how voltage division actually occurs) and verify with DMM that the output voltage is indeed 3.3v. Now you have a nice little way of doing that voltage conversion. Add it to you bag of tricks.
For the newbies, whatever you do DON'T dream of passing ALL your signals over big arrays of optoisolators and voltage dividers, just for elegance sake. What you do is you run multiple voltage "domains" on your board and then when you absolutely, positively have to do the conversion, do so. For example, don't surround a TTL MCU with several optoisolators, run it off of a 5V bus (common ground though) and thread through the single signal (like a UART_TX from the BBB through the optoisolator). On the board level remember you don't necessarily have to thread the UART_RX back a lot of time (like for status). A good example is my PIC based motor speed control...it only receives a TX. You don't have to check at the board level, it always works...saving you a voltage divider back to the BBB.