Layering products can be a real challenge; optimally, though, you'll apply serum first—underneath your moisturizer—so that it can sink in. Then, the moisturizer can go on top to form a protective lipid barrier. But different serums have different consistencies, so ingredients and texture both affect how well a pair of products will combine. Dr. Sebagh Rose de Vie Serum Delicat, for example, is rose hip oil-based, so you can simply massage it in, allow it to absorb, then put your moisturizer over it. Since it's extra hydrating, even a Vitamin A-heavy hydrator like Peter Thomas Roth 'Retinol Fusion AM' Moisturizer SPF 30 will work on top. And as a bonus, it won't aggravate peeling or drying.
On the other hand, if you have a brightening or exfoliating serum like Dr. Dennis Gross Skincare Hydra-Pure Vitamin C Brightening Serum, you'll want to stick to a simpler moisturizer like Kiehl's Ultra Facial Moisturizer SPF 30, since any treatment containing an acid sensitizes skin.