As far as I understand it, the body just uptakes the amount it needs, through the skin, so I just put a handful of Epsom Salts twice a week, in my bath. I also make a cream which I have massaged into my legs twice a week, as follows:-
dissolve a level tablespoonful of Epsom Salts in the minimum amount of boiling water for it to dissolve,
add to it about 5 times more (in volume) of melted coconut oil,
squeeze in the contents of 3 vitamin E gelcaps,
add a couple of teaspoons of a simple type of soft skin cream (I use Diprobase)
You have to keep shaking it though to ensure it mixes well, while it sets.
This is also a good cream to rub in and leave in overnight, on anyone suffering from an itchy scalp, by the way.
Vitamin C, as you say, is best gleaned from fresh fruits and veggies, simple fresh squeezed orange juice, grapefruit juice, or diluted lemon juice or lime juice, berry smoothies are good and blended smooth textured soups made from any greens or sweet potato, or something like that – don’t forget to add Turmeric & Cinnamon (AND make sure you wash absolutely everything!).