Manual
Even if you don’t have any special equipment, you can still start geotagging by doing it manually. If you need a way of geotagging all your old photos, manual geotagging will be your only solution, even if it will cost you a lot of time and will require a good memory.
It basically works by using special software or a web service, in which one photo after another of your archive is shown to you, along with a map. On the map, you then have to manually pinpoint the position from which you took the photo. This is probably not going to be very precise, especially not for those photos which you took on a vacation 10 years ago on a long trip through a foreign country. But especially those photos would greatly benefit from geotagging. If only you’d have already started geotagging back then.
As time advances, I’ll add software recommendations and descriptions of how to use them on this page