First of all, download Win32DiskImager and unzip it.
Execute Win32DiskImager utility, run it as administrator (right click adn select "Run as Administrator").
Decompresses the image (with WinRAR or other program) and verify that your SD Card contains enough space to house the OS.
Select the image you want to flash.
Select the drive letter of the SD card in the device box.
Be careful to select the correct drive; if you get the wrong one you can destroy your data on the computer's hard disk!
If you are using an SD Card slot in your computer (if you have one) and can't see the drive in the Win32DiskImager window, try using a cheap Adapter in a USB slot.
Click Write and wait for the write to complete. Exit the imager and eject the SD card.
Plug into the microSD slot the freshly flashed image.