Nobody can truly explain how the pyramids were built either but there they are!
Line up a few thousand people. Select four that are closest in terms of reach, from fingertip to fingertip. This isn't necessary, but it's helpful. You could probably get away with just picking four people that are the same height.
Cut one rope to the length that you'd like the side of your pyramid to be. Now cut three more ropes to the same length, using the original rope as a rule for each of them. Ropes were made of living things back then and could change length over time, so it might not be a bad idea to cut a dozen or so and let them rest for a week, then select the four that are closest in length. Again, not necessary, but helpful if you want precision.
Have your four people each grab one end of two ropes. Have them walk away from each other as far as they can, until the ropes are taut. Then have them lay the ropes down.
Congratulations. You've marked out the sides of your pyramid at least as precisely as the Egyptians did, and the only tools that you needed were a knife and four ropes.
This could be done with more dimensionally stable rules if you had the time and inclination. Ropes are quick and dirty. I can think of other ways to do it that might be more precise, but it would be a lot more work and the ropes are close enough, so I'd go with ropes.
Precision stonecutting is simple in theory, although it requires some skill in practice. (And they had people that had those skills. And they were not the only ones that had people with those skills.) Mudding lime over the outside also requires skill, one that I could probably demonstrate actually, as we still do something very similar in certain fields. I'm not joking when I say that I could probably lay down a flat (to the eye) surface using only my hands, no tools required, but I do it for a living. The only truly difficult part of the process was getting that many people housed, watered, and fed while construction went on, and this wasn't an insurmountable challenge that required extraterrestrial help. Other cultures did more impressive things. I'd argue that the American invasion of Europe during WWII was more impressive from an organizational and logistical standpoint, as was the Mongolian invasion of basically everything that they could reach that wasn't forested.
Had either of these forces chosen to build pyramids instead, we probably wouldn't be talking about the Egyptians much today.
If you have any other questions about how to build pyramids, let me know. The tough part is getting the people together.
