Download the Ubuntu raspberry pi image from the Ubuntu Server for Raspberry Pi page. Then use the gnome-disks program to copy it onto an SD card. And then wait. Once the image has been written, remove the SD card and then reinsert it (if necessary; about half the time it just mounted the new partitions for me.
The hardware I used to create the cluster consists of: 5 x Raspberry Pi 4B processor cards with 4gb RAM 1 HGST 1Tb HD (repurposed from a laptop) TP-Link 8-port Gigabit Desktop Switch Vanbon Smart USB Charger Cat 6 flat ethernet cables 2 x Short USB C cables Neeyer USB 3.