Thought this might be useful to some people. If like me you have another distro installed to a hard drive and booting via GRUB2, and have been using chainloader to boot your USB Pups from GRUB2, you will find this method does not work with Fossapup64. See screenshot of my 40_custom file below for how to make GRUB2 boot Fossapup64. You want to look at the last menu entry, you'll see my old chainloader puppy entries above.
If you don't know where the 40_custom file is, it was in /etc/grub.d/ on my Debian install - should be same in other distros. Edit the file as root and run update-grub afterwards. You'll need to get your UUID for the drive/flash drive upon which you have installed fossapup64. Open a terminal and type blkid to get this. Change pmedia to whatever type of drive you have installed puppy on. Mine was usbflash.
Hope this helps someone