You need to submit an application in the prescribed format along with income proof such as an ITR, salary certificate or income declaration for the relevant financial year.
You also need ID and address proof, and a declaration about family assets, since EWS eligibility has both income and asset limits.
The application is submitted to the competent authority like the Tehsildar or SDM of your area.
Apply well before your document verification date, since issuance takes time and the certificate must be valid for the relevant financial year.