It can be confusing when applying products - not knowing what goes where. ..But, no matter what, you aren't going to "ruin" anything. At the very worst, your product won't penetrate as deep as it could have. It's OK. :) We do want to show you how to get the very best from each product you have though, so be sure to read on.
Simple Rule: Apply products Thin to Thick.
This allows the thinner serums to penetrate first, and then your creams are thicker - and take a longer time - and then any oils. SPF is always last!
There actually is an order to serums as well. Alcohol or Acid based serums first, then your water-based. If your serums are all the same base, then just apply any thinner ones first - (if you can tell that one is substantially thinner), and then work up in thickness.
Prescription retinoid note - Many prescriptions recommend that you apply them first (on dry skin) and let it sit for a specific amount of time. If you follow this, then once that product is dry, continue on from Step #3.
If you are trying to make your prescription less "irritating". Then, you can layer other products on before you apply it, or you can mix your prescription with a cream or serum to dilute its strength, and then apply to the skin.
Here is a wonderful chart that lists all of our products and the best way to layer them.