Why Vitamin C (aka ascorbic acid)? Because it's a powerful antioxidant, that has the ability to neutralize or even reverse damage to cells caused by free radicals (sun/UV damage) which is believed to be the main cause of our skin aging. Research has shown that topical vitamin C application to skin can stimulate the production of collagen. Normally as we age, collagen degenerates, and as a result, our skin wrinkles. Unfortunately, just ingesting vitamin c supplements will not be powerful enough to boost collagen production, this is why we also want to apply it topically. The good news is that it does not provoke photo sensitivity or phototoxicty which is what causes brown spots (ever have a corona on the beach, unknowingly squirt the lime on your skin, and have to deal with ugly brown spots on your skin for months afterwards? I have, it sucks) so this does not do this. The reason I bring this up is that some may think, "oh I don't have any vitamin c crystals, I'll just apply lemon juice" (which by the way is another good trick, but I can't really use this remedy anymore because it makes my skin more prone to freckles because it causes photo sensitivity).
Now, the tricky thing with vitamin c is that it oxidises very quickly, meaning it's powerful properties that improve the quality of your skin, fizzle out and basically become inactive rather quickly. What this means is that it's pratically impossible to keep this nutrient alive long enough on the shelf waiting for it to reach its consumer, so basically you don't get what you paid for. When I make a batch of homemade vitamin c serum or paste, I really just make it for that one time and make a new batch the next time (this is how do it for maximum results every time, but you can also make a serum, put it in a dark glass bottle and leave it in the fridge to use daily for maybe 4 days, I wouldn't use it longer then that since you're probably just wasting your time). This is why making this yourself is the best way to go if you're interested in using topical vitamin c, and the plus side is that it doesn't end up costing much at all, I bought my vitamin c bottle (vitamin c crystals not tablets) for about 9$ at a health food store, and I also bought my vegetable glycerine (about 5$) there as well, they're very easy to find, these should last you about 6 months.
For a paste I apply to the entire surface of my face (or only certain areas), this is what I do about 3 times a week after my shower and my face is super clean and exfoliated:
What you'll need: Vitamin C crystals (not tablets, it won't work), 100% pure vegetable glycerine, a small glass container, measuring spoons (not absolutely necessary) and a stir stick or substitute.
First step: Put 1/4 tsp of vitamin c crystals in your glass container (if you want to use more put more).
Second step: Pour an equal amount of glycerine in the container and mix with the stir stick. (NOTE: if you have sensitive skin, you may want to dilute this more and use more glycerine and gradually work you're way up to upping the ratio of vitamin c if you don't have any issues.)
Third step: with a dabbing motion, apply to the surface of the skin (make sure it isn't too runny or it will be annoying to have it drip).
For a serum that may last you about 4 days, use more glycerine so its more liquidy, pour in a dark small bottle, and place in the fridge. Since this would be more diluted, you could use this everyday, whereas I only use the paste about 3 times a week (may want to start at once or twice a week) since I do notice a bit of irritation and redness if used too often since I do use a 50/50 dilution.