How am I supposed to install El Capitan on a new SSD without recovery partition?
You said you downloaded the installer. Did you download it to another Mac? If so, mount the .DMG, copy the install file to the Applications folder, then follow this documentation:

https://support.apple.com/en-us/HT201372

Once the new SSD is installed, and you've created the bootable USB drive - insert USB drive into the Mac, start it up holding the alt (option) key, and select the Install El Captian drive.

Format the SSD from Disk Utility, then install.