After purchasing, download your image from the Orders page in your account. Most images ship in QCOW2 format.
Copy the image into your hypervisor's storage location, then create a new virtual machine that points at the QCOW2 file as its disk.
Give the VM sensible CPU, memory and network settings, then boot it. On first boot the image generates unique credentials and SSH host keys.
Connect over SSH using the key you supplied at launch — password authentication is disabled by default.