Process and needed Documents to get H4 stamped in Canada from USA with Active I94 both H1B and H4

USA
1

Hello,
Can you guide me the process and document needed for H4 visa stamping with active i94?
Currently in USA with Active i94 (both H1B and H4). As H1B is already stamped, requesting info on only H4 visa stamping in Canada. Approximate duration to complete H4 stamping and get passport in hand so that I can come back to USA.
In Canada, which US Embassy is quick in processing?

Appreciate your help.

Thank you,
Mano