For DIY body products, should I heat up ingredients or just mix them together?

I'm mixing together ingredients for a specialized lotion and some places I heard you could just mix together ingredients as long as you dissolve the actives in a solvent (like dissolve green tea extract in distilled water), but other places I heard you should heat up the ingredients until they melt and blend them together.

Which one is better?

I would also like to know how to do this.