Once you crack the code, it is easy peasy -- but it's very non intuitive until then. Either use a double boiler (I don't recommend this approach, it makes it harder to tell whats going on, reduces your control and makes setup feel like a chorae) ... or buy a few dozen eggs, a couple pounds of butter and a dozen lemons and just practice the sequence until it clicks.

The key is to control the temperature carefully, and keep that temperature homogenous and even... that means knowing how warm and cold your ingredients are, and steady whisking.

Two ways to do it:

• Whisk together eggs, water and lemon juice until the mixture thickens, and then add melted butter slowly (your slowest and most foolproof method)

• Whisk your eggs to aerate them, set them aside. Melt your butter, remove it from the heat and add your (cold) lemon juice and water. Should be about room temp now. Whisk it together and drizzle in the eggs, whisking constantly. Then put it back on the heat and whisk it steadily till it thickens, which will be quite soon.

The first path is the correct way, in that it minimizes the risk of putting the eggs into a hot pan (and curdling them), but it's also slower and more involved. Basically, any way that ensures the eggs are about the same temperature as whatever gets mixed into them, and heated up gradually from there, works.

Sous vide can make this easier

The key is heat control. You need the butter barely melted when you mix in the egg yolk and you need to mix everything together before the egg yolk cooks by itself.