I got few advices which I hope will help you :)
1. Check your routine first - are there other actives? Any active that might work badly with vitamin C like retinol or acids?
2. Choose formula - usually brands won't tell you the pH of the product but some forms works only in pH around 2-3, other forms can work in pH around 5 or even 7. Majority of brands work on ascorbic acid or other forms like ascorbyl glucoside, SAP, MAP, or EAC - EAC is probably the most powerful but also most expensive version of Vitamin C (still, the best one for sensitive skin)
3. Since Vitamin C boosts UV protection it's good to introduce Vitamin C during the morning routine
4. Start easy - 2/3 times per week, and then more often. Depends on your skin reaction and needs.
5. It's good to mix Vitamin C with Vitamin E, niacinamide and ferulic acid - search for such substances in your skincare or vitamin C product.
6. Make sure your vitamin C is fresh - especially ascorbic acid likes to turn colour from white/pale yellow to brown in short amount of time.
7. Even if Vit C boosts UV protection we still need to use sunscreen.
8. Don't be surprised when the cream you use after using it on vitamin C turns into water - it can happen :)
I hope these advices will help you, I also added some product recommendations - vitamin C products that worked for me :)