You should use your vit c serum in the morning. Do not put your vit c in hot oil.
Google vit c serum recipes. You will find lots of them. There are ones made with oil and made with water. I use a water based one. It absorbs very quickly. The recipe I use isn't mine so I don't feel I should post it.
Some people just put L-aborbic acid in water mix it daily. That is about as simple as you can get.
Give it a google though and you will have lots of recipes.