I struggled the same with another ES6-first library. The best way to convert those to ES5 is via copy / pasting the code in https://babeljs.io/repl/. After a while, you'll quickly learn the syntax and the code will come naturally.

Why not write in ES6 and transpile downwards to ES5? You're just fighting the direction of the wind otherwise. ES6 feels much, much nicer to work with in nearly all ways.