How to use Nikon D5100 pop-up flash to trigger Yongnuo speedlight?

New to using off camera flash… I have a YN560-II… Trying to figure out how to use the flash without placing it on my DSLR. Would like to use the camera flash to trigger the speedlight.

Put the YN560 in the correct optical slave mode. There are two of these modes, i'm not sure which one is the right one for this application. One is for instant triggering, the other for delayed triggering to be used with Nikon CLS (which your camera does not have)

Activate the popup flash on your camera, set a flash exposure compensation to mark it down so it doesn't register that much on the final photo, and fire. Also try to get the red sensor of the YN560 pointed towards you if that is possible.

Optical slave triggering works best inside closed rooms. Outside they often fail to get triggered as they don't register the flash due to lack of reflective surfaces around them and a high ambient light level.