You'll need to set up a double boiler. That's a big pot you can boil water in with a metal bowl that can sit over the top of (but not touching) the boiling water. Fill the pot about one-third of the way with water and turn it on high to get a boil going. Put all the ingredients in the metal bowl and whisk together. Put the bowl over the pot of boiling water, turn the heat down to medium and stay with the bowl for a bit, whisking to get things warmed up.
The first thing you are looking for is for the sugar to dissolve and the butter to melt. Once the mixture is homogenous, you can whisk a bit less, but occasional stirring is important throughout this entire process. Scientifically speaking, what you are trying to do is gently cook the egg proteins to thicken this mixture. If you cook them slowly, keep stirring and allow them just to thicken, you will have nice, soft, thick, yummy lemon curd. If you try to hurry the process, you run the risk of cooking the eggs too fast and then you will have scrambled eggs. So it's better to go slowly, take a bit more time and get it right. This process can take quite a while. Don't worry, just don't go far from the bowl and continue to whisk every few minutes.
Eventually you will see the liquid get thick and become a bit "gloopy." Here is a test to tell you when it's done: Stir the mixture with a wooden spoon. When you pull the spoon out of the lemon mixture, run your finger across the back of the spoon. If your finger leaves a line on the spoon, without the curd running back across, the curd is done. If the curd is still loose, or runs across the spoon and fills in the line you drew, just return the bowl to the double boiler and keep cooking it until you achieve the desired texture.
Once the curd is done, one of the things that is important to do is to stop the cooking. In order to do this, fill another bowl with ice. It needs to be bigger than your curd bowl. Put the bowl full of curd on top of the ice and gently stir the curd to help it cool off. As the curd cools, the egg proteins will solidify and you'll notice the curd getting thicker. Keep the curd over the ice and stir occasionally, gently, until the curd is completely cool.
Once it's cool, it's ready to use. Curd keeps for a few days, always refrigerated.
Makes 4 cups