There was a NOVA episode just a few weeks ago that demonstrated exactly how people could do this with very simple tools! (Like, stick in the ground simple.)

The records they were sharing and recreating were from the last 1,000 years, but it's not appreciably different from what Eratosthenes did, and what we assume older calendar-making societies did. And they demonstrated so beautifully exactly how it works, A+++ go watch the show and then plant your own stick in the ground to measure sun things!