The flavorings for the sugar syrup is optional, though it is always good to use at least rosewater.
The sugar syrup has to be at room temperature and needs to be poured on the hot cake, to absorb completely. You can prepare the syrup in advance and keep in the fridge, and bring it back to room temperature.
Resting the batter in pan is not mandatory, though it will help the semolina absorb the liquid in the batter and hold good shape once baked.
Make sure to spread the batter thin - the basboosa shouldn't be over 2 inch thick.