Serum - when to use?

[sociopathsunited]

I use the Superdrug Vitamin E range - hot cloth cleanser and then either day or night cream.

I've just bought a wee bottle of the serum from the same range. Should I use this before the night cream, or before the day cream, or both? Or do I use it instead of one of them?

Usually before one or both. Sometimes I use instead of day cream in summer, especially if I'm also applying SPF)tinted moisturiser.

Thank you