LeMaker offers several predefined images for download. These can be found at http://www.lemaker.org/product-bananapro-resource.html. Simply download the image file you want and copy it to a microSD card. The minimum required SD card size is 4 GB.
When using the Windows OS, WIN32 Disk Imager (http://sourceforge.net/projects/win32diskimager) can be used in order to transfer an image to an SD card. However, make sure that you select the correct drive for the Win32 Disk Imager. An incorrect drive may result in loss of data.