Create a bootable USB flash drive with OS X 10.11 on it, after downloading it on a computer that does have internet access. Then, stick the flash drive into the old Mac, and boot from it (hold Option at startup and select EFI volume).
When it boots, it will prompt you to install OS X. First go into Disk Utility and format the SSD with the "Mac OS Extended (journaled)" format. Then, select the SSD as the destination for your installation.