Okay a few things: Don't buy Vitamin C serums from places like Sephora because they need to be kept chilled to keep fresh.
There are a few different ways you can go about this-
1. Buy this kit ($15)- it gives you all the ingredients to mix when you get it, and then you keep the bottle pump in the fridge. Some don't like it because of how it feels once it dries on your skin (you can still feel it, it's a tiny bit tacky), but i put it on right before I go to bed and it really does work. Link:
http://www.skinactives.com/Vitamin-C-Serum-Ascorbic-Acid-L.html
2. MAKE YOUR OWN. First (and easiest) way (and this is the way i went when I first started using them). Buy the Vitamin C- there are many places you can get it super cheap, but I prefer Skin Actives. This $5 will LAST FOREVER:
http://www.skinactives.com/Ascorbic-Acid--L-Vitamin-C.html
What you'll do is mix enough to either keep for a month or so or I just mixed a tiny bit each day. The ratio of powder to water depends on your skin. Everyone should start at about 10% and work their way up unless they're sensitive, in which case start at 5% consistency, SO DO POWDER TO WATER AT ABOUT A 1:9 consistency. You mix it in warm water to let the crystals mix fully in the water then let it cool and put it on your face, or if you've made enough to last more than a night, put it in pump (I can provide links for where to get those for cheap, too) to put it in the fridge. Now, obviously, this "serum" is going to be more like... water. I used a q-tip to apply it, and I PROMISE IT WORKS.
To make it more serum-y, you can add glycerin (article on how to do that:
http://healthbeautyandskincareproducts.com/how-to-make-a-homemade-vitamin-c-serum)
Buy glycerin here for $2.50 (REMEMBER, YOU DON'T *NEED* THIS):
http://www.skinactives.com/Glycerin-Plus.html
When you guys are ready to make your own Vitamin C + Ferulic acid and jojoba + Vitamin E, come back to me!
Again, if I'm making this sound complicated, go with step 1. But it's not. Just buy the crystals I posted in step 2 (there are other places you can buy them, all about the same cost, and just to put this out there, I'm not associated with that site whatsoever) and mix with warm-hot water at a 1:9 ratio for 10% or 1:19 if you have really sensitive skin. Let it cool, apply with a q-tip after you've applied your nightly moisturizer. If you made more than a night's worth, bottle it up in a dark amber bottle like this one, which costs less than a buck:
http://www.abundanthealth4u.com/2_oz_Amber_Bottle_with_Dropper_Cap_p/9121.htm
KEEP CHILLED. THESE ONLY LAST ABOUT 3 MONTHS, and you'll know when your homemade has gone bad because it'll turn orange-y, which is what makes the brands at Sephora that aren't chilled and are already orange so suspect.
If you want to explore further, use this forum and search, that's where I learned everything I know, and I'm sooo glad I discovered these!:
http://www.essentialdayspa.com/forum/viewthread.php?tid=39367