The best way if you have the option is to get a larger SD card (32-64gb), and use the option "Format as internal storage".
If you have this option, it will combine internal storage space with the SD card, but you cannot remove the SD card after. However it will be your existing (8gb) + whatever you put in (32gb) usable as a single block of 40gb.
If you don't format as internal storage, you will not be able to store apps on the SD card.