How to Install the Raspberry Pi OS

Method 1 - Use Raspberry Pi Imager to flash an image from the cloud

Download imager from here

Method 2 - Manually install an operating system image

(for slow internet connections)

Download a copy of the required Raspberry Pi Image from here.

Flash to an SD card with an SD card flasher app such as Balena Etcher found  here

A word of warning !!
Make sure you do not have any other sd cards plugged in so as not to mistakingly choose the wrong card and write all over your precious data !!

If you have flashed any of the 'Raspberry Pi OS with desktop' images you can connect the device to a monitor,keyboard & mouse and boot the Pi up.
I personally recomment connecting via a KVM switch. This way you can quickly flick back and forth to your pc for any reference material such as this you are following.

Once booted you will get a 'Welcome to the Raspberry Pi Desktop' spash screen and from there you can fill in network credentials and other parameters such as Country,Language & Time.

However if you have flashed any of the 'Lite' versions which have no desktop UI, the device does not know your network detail so you are unable to SSH into it.
fortunately there is a 'trick' you can do to get these details onto the card before the 1st boot

This method to setup WiFi must be completed before you boot this card for the first time. This is the point at which the system checks for a file called  wpa_supplicant.conf . If you have already booted the card you will need to re-write with a fresh image and continue.

create a file called wpa_supplicant.conf in the /boot folder of the sd card replacing 'MyNetworkSSID' and 'mynetworkpassword' with your own credentials.

Also at this point create a blank file named SSID in the same folder. This will trick the OS into enabling ssh so that you can connect to it from another computer.
When the Pi boots, it looks for the ssh file. If it is found, SSH is enabled and the file is deleted.

country=uk
update_config=1
ctrl_interface=/var/run/wpa_supplicant

network={
 scan_ssid=1
 ssid="MyNetworkSSID"
 psk="mynetworkpassword"
}