Cook over simmering hot water ( WHISKING the whole time so it don't curdle ) for 10 minutes or longer ; it will become the consistency of thick pudding. You can use a thermometer and measure the curd , it should reach around 170°.
Note: It may take longer than the 10 minutes of stirring time , try getting the curd to the same simmering temp before you start timing , because sometimes when you use a double broiler its hard to get it up to the proper temp.( I don't always use a D.B. just don't bring it to a boil or let it scorch if using direct heat.)