Step 1: Create CSR from Plesk panel:

1. Login to Plesk Panel.
2. Click on domain.
3. Click on SSL/TLS certificates.
4. Click on +Add SSL/TLS Certificate.
5. Enter the details. You can refer to below screenshot.
6. Click on request.
7. The CSR will be generated.

Step 2: After CSR generation provide the CSR to SSL certificate provider. They will provide you verification method to complete verification process.

Step 3: Once you have completed the verification process, the SSL provider will provide you with the SSL certificate file.

Step 4: Uploading SSL certificate file in Plesk panel.

1. Login to Plesk Panel.
2. Click on domain.
3. Click on SSL/TLS certificate.
4. There will be your domain name >> Click on that.
5. There will be option of upload certificate >> Upload the certificate  and CA bundle.

Step 5: Once you have upload the certificate files you need to enable SSL/TLS support.

1. Click on Hosting setting in your Plesk panel.
2. Under security section enable SSL/TLS support  and Permanent SEO-safe 301 redirect from HTTP to HTTPS.