You can use a tuning fork, but that doesn't matter. Temperament is about the tuning relationships between notes of the scale. In equal temperament they're slightly enharmonic, ie adjusted away from ideal harmony in order to allow transposition into different keys. This doesn't sound as pretty, but it's 'equally bad' across all keys and we're used to it. If you tune strings against each other then the intervals are consonant. Your starting pitch can be rather arbitrary as long as you're not trying to match it to any other instrument. If you are trying to match to another instrument (in ancient times, a horn was the most likely candidate) then you blow a note on that and tune one string to it.

Musical key intervals are based on simple mathematical ratios, which haven't changed in the interim.