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.
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 stay lit up at the same time. You can then power down your BeagleBone Black.