Indexing documents into Elasticsearch using the NEST Elasticsearch .NET client can be performed through various methods, ranging from indexing single documents to handling multiple documents with advanced techniques. For single documents, the IndexDocument and IndexDocumentAsync methods provide straightforward approaches, while the fluent or object initializer syntax offers more control. When dealing with multiple documents, using the Bulk API is more efficient than iterating through individual index calls, and the BulkAllObservable helper further optimizes this process by managing retries and batching mechanics. The blog post also discusses creating ingest pipelines to manipulate and enrich data before indexing, utilizing features such as the ingest-geoip plugin to convert IP addresses into human-readable locations. For large datasets, considerations like partitioning collections into smaller batches and adjusting timeouts are necessary to avoid errors during bulk operations. Overall, the NEST client offers multiple configurations and helper functions to facilitate efficient document indexing in Elasticsearch.