Qemu Installation comes with some handy HTML documentation check if you have that, if not you can look for an online version at the Qemu official website.
as for the way to boot from a live CD ISO image this is the command syntax:
qemu -boot d -cdrom <your-iso-image.iso> -m <ram>
<ram> parameter is used to tell qemu how much memory to dedicate to your guest system from the host System.
If you wish to use create a hard disk image and associate it with the qemu VM as well (useful when formatting the VM using the ISO) you can execute these two commands:
1) First, create the hard disk image:
qemu-img create <disk-image-name.img> <size-ex-25G>
Pass in the your desired disk image name, and the size you want to allocate for it.
2) Now we can use both Iso and Disk image to boot:
qemu -boot d -cdrom <iso-image.iso> -m <ram-in-MB-example-512> -hda <disk-image.img>
That would be it.