They didn't.

They didn't like go out on Dec 21st, and look where the sun was and mark it. They didn't even have calendars like that. They watched the sun every day, and waited until it stopped being lower in the sky at it's highest point in the day (or whatever other sign of the solstice they wanted to use), and marked that angle and built whatever viewport they wanted (a door, tunnel, etc).

Then they could just go wherever they built the thing that pointed at that point in the sky, and go, oh, okay, the solstice is soon, or just happened, or whatever and plan accordingly.

It actually wasn't really accurate to the day, anyway. There are a few days on either side of the solstice where the effect is basically the same for the viewer.

Something to keep in mind is that this isn't only useful for determining the exact date of the winter solstice, which they may not have even cared that much about. You can see roughly where you are in the year on either side of the solstice by looking at how far out of alignment the sun is on a given day. So it could be useful throughout the fall and even well into the winter for gauging the passing of time. People didn't need to plan day by day or even week by week, but they did need to do things in roughly the right part of the year.

People act like this is some unexplainable advanced technology, and anybody can just do this with a stick and some rocks.