From your live stick, bevor install, do

# cp /dev/your-disk /your/image.img

or to save space:

# gzip < /dev/your-disk > /your/image.img.gz

or faster but might need to install it first:

# lz4 /dev/your-disk /your/image.img.lz4

To get a list of your disks, there’s lsblk.

I recommend partition-wise backup, it’s always a bother to extract/shrink individual partitions in images of whole disks. That would be /dev/your-disk1 etc