Install OS the first time

If your Mac previously had macOS 10.12.6+ installed, then you can boot into Internet Recovery Mode using either Command + R to boot the online macOS installer for the OS that was installed on your old SSD or use Command + Option + R to access the online macOS installer (in theory this should work, but some Macs will only boot to the online installer for the original OS for your Mac).

Use another mac to download macOS High Sierra from here,

use your Safari browser, others won't work,

How to get old versions of macOS – Apple Support

Go to Download OS and click on macOS High Sierra

this will redirect you to the App Store High Sierra download page.

It should say Get or Download.

This downloads Install macOS High Sierra.app to the Applications folder.

Now you make a bootable USB installer read the instructions here,

How to create a bootable installer for macOS – Apple Support

Once you have created the bootable USB plug it directly into

the mac with the new SSD.

Restart the mac while pressing and holding the option/ alt key

in a minute or two you will see the Startup Manager, select the USB

and press Return.

When booted to the USB click on Disk Utility and press Continue.

Click on view in the menubar and select Show All Devices.

Highlight the Disk not any indented Volumes the Disk is usually named

after the manufacturer, press Erase.

Give the Disk a name.

Format: APFS.

Scheme: GUID Partition Map.

Click Erase.

When Done quit Disk Utility.

Click on Install OS press Continue

the installation process should start up, follow the prompts.