How do you fix the problem of egg curdling as you make lemon curd?

If your recipe is like mine, you have a thickened lemon mixture that you add egg to. Put some of the hot mixture in the beaten egg stirring it all the time. Add about a third of the mixture to the egg to warm it up and then add that mixture back into the rest of the lemon mixture. Stirring all the time so that the egg will not cook in lumps.