If one is beneficiary of a cap-based H1B approved under FY2019 quota, s/he may apply for a H1B visa based on the approval. Most of the U.S. embassies / consulates in the world accept applications for an H1B visa up to 90 calendar days ahead of the start date mentioned on the approval notice. It is important to know that once approved, the visa is not valid for travel until 10 calendar days prior to the start date indicated on the approval.
Example
If a beneficiary has a H1B petitioned approved under the FY2019 cap with a start date of October 1, 2018, then s/he can apply for a H1B visa based on this approval on or after July 3, 2018. If the visa is approved, then s/he can travel and present him/herself at the port-of-entry for admission on or after September 21, 2018.
In India, the U.S. embassy in New Delhi and the U.S. consulates in Mumbai, Kolkata, Hyderabad, and Chennai accept H1B visa applications up to 90 days prior to the start date indicated on the approval notice.
Procedure to apply for H1B visa in India can be found here.