Follow this thorough guide to get your own low poly scene in Unity.
Justinas Telksnys has published a video guide on speed level design in Unity. The artist guides his viewers through basic steps towards the final atmospheric low poly scene.
The young talent basically used three packs to set up the low poly environment:
Justinas sets up the initial landscape, works on assembling the assets and vegetation, edits the lighting and follows other simple steps, which are typical of any production process. The video guide can really help you in studying low poly art in Unity.