Creating new mappings can be done using the put_mapping API. When a document is indexed with no mapping associated with it in the specific index, the dynamic / default mapping feature will kick in an automatically create mapping definition for it.
Mappings can also be provided on the node level, meaning that each index created will automatically be started with all the mappings defined within a certain location.
Mappings can be defined within files called [mapping_name].json and be placed either under config/mappings/_default location, or under config/mappings/[index_name] (for mappings that should be associated only with a specific index).