Custards are done when a knife inserted into the center come out clean with no milky residue, and they are overcooked if the top is puffed, bubbly and tough. That way, if you break an egg yolk accidentally into the white, you can discard just the one egg instead of having to start all over again. Cold eggs are easier to separate than room temperature eggs. Make sure there is no trace of grease or yolk in the eggs or on the bowl or beaters. The bowl and beaters should also be bone dry.
Copper helps stabilize egg whites without cream of tartar.
Start beating slowly, then increase speed when the whites become foamy. For souffles, I beat the whites just until they hold soft peaks when I lift the whisk. Stiff, not dry whites are shiny, smooth, and hold a perfect peak on the end of the whisk even when it is lifted straight up. If the peaks look grainy, they have become dry. Rescue them by adding another unbeaten egg white and trying again, paying closer attention this time.