How to charge a newly bought Nexus 5X(lithium polymer)?

Charging a new battery for x hours before use is a "legacy hangover" from the days when Nickel Cadmium batteries were used. Those batteries had a "memory effect". As a result, over a period of time, they "remembered" lesser battery capacity than rated capacity. To prevent this, it was advised to always fully charge these batteries

This is not the case with Lithium Ion or Lithium polymer batteries. There is no logic to charge fully or x hours before use. Just use the battery normally, as suggested by others in comments

Table 1 corroborates with additional information