To start, make sure the BeagleBone Black is powered down, and unplugged from the power source.
Now, insert the microSD card into the slot on the back of the BeagleBone Black. It should snap into place.
Now, find the "User Boot" button. it's located on the same side of the BeagleBone Black as the microSD card slot, and is the only button in that area. See the below screenshot (from beagleboard.org) highlighting the 'User Boot" button.
Hold the "User Boot" button down, and then plug in the power (USB or 5V adaptor). Keep holding down the button until you see the bank of 4 LED's light up for a few seconds. You can now release the button.
It will take anywhere from 30-45 minutes to flash the image onto the on-board chip. Once it's done, the bank of 4 LED's to the right of the Ethernet will all turn off. You can then power down your BeagleBone Black.