Technically we didn’t evolve from monkeys. Neither did we evolve from apes such as gorillas and chimps ( easy difference between ape and monkey is apes don’t have tails). Instead, humans and apes share a common ancestor that lived millions of years ago. This common ancestor diverged over time to form many different species of hominid (a technical term for great apes), only one of which survived to become modern humans.
Genetic studies show that primates diverged from other mammals about 85 million years ago. The earliest documented members of the genus Homo (we are Homo sapiens) are Homo habilis, which evolved around 2.8 million years ago; the earliest species for which there is positive evidence of use of stone tools.
This change from our common ancestor into us would have been very slow taking 10’s of thousands of year. Lots of little changes over a long time would eventually lead into something very different from the original.