1. For knowing when to toss products after opening: Jenny Chang / BuzzFeed Two things to keep an especially close eye on? Acne treatments because their active ingredients lose effectiveness over time. And sunscreen, which can essentially go bad once its exposed to a temperature change. Read more: It's Time To Start Throwing Out The Makeup You've Had For Years 2. For the order of operations: dermstore.com There are always exceptions, but a good general rule is remembering to apply your products from thinnest to thickest consistency.Read more: In What Order Do I Apply My Skin Care Products? 3. And for how much — approximately — to use for each step: makeup.com Less is (sometimes!) more.Read more: How Much Product You Really Need — A Guide 4. For mornings when you need a streamlined skincare routine — before moving on to makeup: wakeupformakeup.com Don't forget the SPF too, though. ☀️Read more: Face Products & The Order of Application 5. For understanding all things acid-related: goalstogetglowing.com AHAs and BHAs both act as mild exfoliants — but it's important to know the key differences between both.Read more: Everything You Need To Know About Acid Exfoliation 6. Plus, a further breakdown of common ones: AAD 7. For product timelines and effectiveness: birchbox.com Shoutout to my fellow ~impatient~ people. 👋Read more: How Long Skincare Really Takes To Work 8. For deciphering all those symbols on the packaging: Zoe Burnett / BuzzFeed 9. For a useful primer on oils: glamour.com Also, if you're not sure where to start with oils, here are several that our readers swear by.Read more: Glamour's Everything You Need To Know About Oils 10. For diving into Korean-style skincare — and its famous 10-step routine: theduetglow.com It's definitely not a quick routine, but after trying a K-Beauty regimen for a month straight, we actually loved the results.Read more: I Tried A Korean Skin Care Routine For A Month And This Is What Happened 11. And for answering the inevitable "But what *is* that, though?" question when it comes to step #5: eminenceorganics.com Read more: Everything You Need To Know About Essence, The Asian Beauty Secret 12. For that time of the month — and how it might affect your skin: eminenceorganics.com Read more: What to Know About Skin Changes During Your Monthly Cycle 13. And for suggestions on how to adjust your routine if you'd like: eminenceorganics.com 14. For all those power combos we keep hearing about: byrdie.com Some products are even more effective when paired together — while others should always stay separate.Read more: What To Know About Combining Skincare Products 15. For the (sometimes surprising) culprits behind breakouts: cosmopolitan.com (I'd also add: Wash your pillowcases and towels frequently!)Learn more at Cosmopolitan. 16. And for your next DIY at-home spa day: sheknows.com Read more: Easy DIY Beauty & Skin Tips To Try At Home What's the most useful skincare tip or product you swear by? Share it in the comments!