The first and most important thing you can teach your baby is that Mummy Comes Back. All a baby will take from being left to cry for a long time (20+ minutes) is that there's no one to help them.
That said, it's absolutely fine to let them get on with it in a safe place (cot, moses basket, bouncy chair) for a couple of minutes while you fetch a drink/food or go to the toilet. Or even just while you stick your head out the back door and breathe for a minute. It's leaving them for ages that isn't helpful.
If in doubt, feed. If it's not that, change their nappy. Still upset? Take off or put on a layer of clothing - feel the back of their neck, not their hands, to judge. If they're still upset, try food again. If it continues for many many hours, ask for help - don't let anyone make you feel like a paranoid freak for worrying about your baby, that's what parents are for and it's completely normal. Better to ask stupid questions than ignore something that later becomes a problem.
Late afternoon was known as Grump O'Clock in our house. That's when ds was over-tired, over stimulated and getting geared up for the evening cluster feed. Be prepared for this, and make sure your partner is too - it coincides with many partners getting home from work, mine felt really guilty for a while thinking it was his fault but actually a couple of oz of expressed milk and some skin to skin with daddy was really nice and meant I could do grown up things like cook dinner.
This is all from the grand total experience of 1 fairly easy baby who is now 15 months feel free to ignore anything that doesn't suit you - as you should do with all advice, both asked for and foist upon you