So I upload the ISP then without unplugging the usb, plug a 120 ohm resistor
I would unplug the board after uploading Arduino ISP before adding the resistor. It's much safer to disconnect power when mucking with wiring.
(Does it have to be a 120 or can it be any resistor?)
I have no idea. You don't want to use anything below 120 or the current flow gets to be too much. If the resistance is too high, the auto-reset circuitry will be able to pull the line low and cause a reset. If you don't have a 120 on hand, pick the next higher resistance and try it. You won't cause any damage, it just may not work. You can use more than one resistor (like two >= 240 ohm resistors in parallel).
directly from the 5V to the reset
Yes. The RESET line needs to stay close to +5V.
then I pull out the resistor
No. Once the Arduino ISP Sketch is installed, the resistor stays in place until you are finished using the Arduino as a programmer. There is no need to yank the resistor at any point in this process.
(How do I know whether or not the auto-reset is disable?)
The bootloader flashes the LED on pin 13. The bootloader will run once when the board is first powered (LED will flash). If you have auto-reset disabled, when you open the Arduino serial port using something like Serial Monitor, the bootloader will not run.
Sorry for all the question, I just don't want to short something that's not suppose to be shorted
Don't use a resistor less than 120 ohms. Only muck with the wiring when power is off. Other than that, your Arduino is safe.