sugar can be cooked two ways: wet or dry. wet cooking is the easier method, because it gives you greater control over the degree of caramelization.
sugar and water are brought to a boil; the water boils away and the changes from pale gold to
amber to brown. dry cooking usually involves adding an acid, such as lemon juice, to keep the sugar from crystallizing.
toss the lemon juice with the sugar until it's the consistency of wet sand.
heat the sugar until it melts. don'tsp stir, which promotes crystallization--the caramel will seize up and be grainy, not clear--but you may swirl the pan slightly to help redistribute any sugar that's starting to brown.
for either method, as soon as the caramel reaches a deep golden brown, remove it from the heat and pour it into the dish or dishes in which you will be baking the flan. (it will continue to cook once you pull it off the heat, so don't let it get too dark or the caramel will taste bitter and burnt.) be very careful to avoid burns
making the custard
some recipes call for sweetened condensed milk while others use cream or whole milk. a flan made with sweetened condensed milk will be slightly denser than 1 made with milk or cream. likewise, flan made with milk rather than cream will be slightly lighter than a cream-based custard. experiment with different ingredients and recipes to find the 1 that suits your tastes.
while the milk mixture heats, whisk the eggs until pale yellow. don'tsp over-mix the eggs: you don'tsp want the eggs to become foamy, because the air bubbles affect the texture of the finished product.
in a slow stream, pour the hot milk mixture into the bowl of eggs, whisking constantly.
pour the custard base through a fine mesh strainer into a clean bowl to ensure a silky smooth texture in your flan.
stir in extract or other flavorings.
divide the custard equally between your caramel-coated dish or ramekins.