Namely, what pediatrician Harvey Karp calls the “5 S’s” of (1) swaddling the baby, (2) holding the baby on their side/stomach, (3) loudly shushing in the baby’s ear, (4) swinging the baby back and forth, (5) giving the baby something to suck on.
Go find Dr. Karp’s “happiest baby” video or other videos of people performing these steps, as text posts don’t really convey the idea fully.
These techniques are extremely effective on the vast majority of babies (usually just shushing and swinging is enough for my current newborn), but many new parents don’t know about them or do them improperly. In particular, shushing needs to be a very loud constant white noise, and the rocking/jiggling/swinging of the baby should be a bit more vigorous than new parents might naturally do.