Claiming Spanish citizenship through abuelo

My father's birth certificate shows that his father (my grandfather) was a Spanish national. He immigrated to the Philippines a long time ago. My grandfather is now deceased. I hold a Filipino citizenship but I would like to claim Spanish citizenship through my grandfather. How do I go about doing this? Do I need to get my grandfather's birth certificate as proof of my lineage? You response on this matter is greatly appreciated.

First of all you need to find out in which year your grandfather left Spain and which were the reasons. This is important in order to determine whether you can opt for Spanish citizenship via' La Ley de Memoria Histórica' (which will enable you to obtain Spanish citizenship without having to reside in Spain previously) or if you will need to reside in Spain for one year before you can claim it.