Going trough Kotlin by example.
Here is an overview over the basic syntax, like a cheatsheet: https://kotlinlang.org/docs/basic-syntax.html. Common idioms can be found here: https://kotlinlang.org/docs/idioms.html.
Direct comparison of Java vs. Kotlin code: https://fabiomsr.github.io/from-java-to-kotlin/index.html
To get some more practice, here are some CodeLabs from Google: https://codelabs.developers.google.com/?cat=Android&product=android