To make larger or smaller quantities, one can multiply or divide the ingredients. I happen to have a 15 liter jar, so I multiply all the ingredients by 3 to provide a good fit.
Fruit alcohol is widely available in most large food stores (supermarkets). It is a clear alcohol without much taste, which enables it to absorb the taste of the fruit. There are several different brands in France, but they all come in a 1 liter bottle with a simple label along the lines of "Alcool pour fruits". They are available in the alcohol section of the store and typically have 36% or 40% alcohol.
Since the recipe includes the peel of the orange, my personal preference is to try to use an organic orange (to avoid traces of pesticides).
If you don't have a walnut tree, you can try the local forest or ask a neighbour. The walnuts should be green on the outside (typically around the end of June).
I can never remember to shake the jar daily. I'm doing well if I remember once a week. However, having following this recipe several times and always had a good result, this step doesn't seem to important. I've also left the mixture for far longer than 40 days (up to a year) with no problem, so it seems 40 days is a minimum but it is OK to leave it longer.
After the juice is bottled, the taste improves if one leaves it for several months before drinking. One should store it in the same place as ordinary wine is kept, as the ideal storage conditions for Vin de Noix are the same as for ordinary wine. I find it keeps well for several years.