For the classic cobbler: THERE IS A NOTCH IN THE HEADER MAKE SURE ITS RIGHT NEXT TO THE PIN LABELED #21!
For the T-cobbler: THERE IS A NOTCH IN THE HEADER MAKE SURE ITS RIGHT NEXT TO TWO HOLES AT THE T JUNCTION
Its really important to line up the header right, because otherwise the pins will be reversed, making the cobbler useless
Repeat for all 26 pins!
Then place the cobbler PCB on top so that the short ends of the header stick through the 26 pads on either side of the connector. If the PCB doesn't slip right on, check that you have the pins aligned and spaced right. It doesn't matter which way the PCB is oriented on the breadboard.
However, it is possible to put the cable in upside down on the Raspberry Pi! Check the images on the left to verify that the stripe is on the edge closest to the SD card!
Note that the stripe may be red (gray cable) or white (black cable)