The steps to make UEFI USB boot stick are as follows:

The steps to make a UEFI USB boot stick are as follows: [https://www.linuxquestions.org/questions/slackware-14/easiest-way-to-make-a-current-uefi-boot-usb-4175551386/]

1. Backup everything on the USB stick. All data on it will be deleted by following this procedure.
2. Using gdisk, delete all existing partitions.
3. Still in gdisk, create a partition 100 to 300 Mb in size.
4. Partition the rest of the space as you desire. (I'd probably make it all one partition).
5. Set the partition type of the first partition to "EFI System partition." (code EF00)
6. Set the partition types of the rest as you like. "Linux data" is code 8200.
7. Write the changes to the partition table and exit gdisk.
8. Format the first partition: mkfs.vfat /dev/sd?1
9. Format the second partition: mkfs.ext4 /dev/sd?2

The stick is now ready. All you have to do is copy the stuff onto it. Fortunately, everything you need is in the Slackware file tree.

My preference is to use Alien Bob's USB boot image & root filesystem from the "usb-and-pxe-installers" directory. To do this:

1. mount -o loop /path/to/slackware64-current/usb-and-pxe-installers/usbboot.img /mnt/loop
2. Copy everything from your mountpoint (/mnt/loop in my example) exactly as it is to the EFI system partition on our boot stick
3. Now, provided that you have a file called /path/to/media/usbstick/EFI/BOOT/BOOTX64.EFI on the stick, it'll boot.