Free Let's Encrypt SSL installation can be done from Direct admin panel .You may please follow the below given steps to install the free SSL certificates on your domain .
1. Login to the Direct Admin panel.
2. Select "SSL Certificates" option under "Account Manager".
3. Select the option "Get automatic certificate from ACME Provider".
4. Select the domains on which you want to install SSL certificates.
5. Click on save.
Once this is done, you can check for the SSL Certificate details in https://www.sslshopper.com/ssl-checker.html .