The process was absolutely nothing like a modern election. Instead, the greatest princes of the realm, the seven prince-electors (3 ecclesiastical and 4 secular), voted. Anyone could theoretically be elected Emperor, but in practice the title remained between a few families. The process was often more of a rubber-stamp, but each new emperor would have to bribe his way into confirming his or his son's succession. This made the emperor beholden to the great men of the kingdom. Throughout it's history, the title was often monopolized by one dynasty, even though each successive emperor was technically elected.

This is a description of the process from the mid-14th century onwards, before that the elective nature of the title was rather a gray-zone, as theoretically all early medieval kingship had an elective nature.

There has been a lot written about this, look it up on Wikipedia for a more detailed description. I got this information from Fredrick Heer's "Holy Roman Empire" as well as a plethora of other things I've read on the subject.