Mix the ingredients (except the lemon juice) the previous night and put it into the fridge. In the morning add the lemon juice slowly, tasting it time and time again till you reach the tartness you like.
Notes
I usually forget to make it the night before, I'm not so organised (I just eat what I feel like eating in the morning) so I use hot water. By the time the mixture cools down, it becomes a pudding. I give the lemon juice to it this time. The only disadvantage of this method is that your pudding is not going to be as cold as it would if you made it the night before. Although in winter it can be an advantage ;-) Or for those of you, who get up early but eat her berakfast late (like me), it can cool down completely. I had a yummy Matcha tea with my pudding to wake up :-)