First, ensure the USB flash drive you intend to use for multi-boot is inserted into a USB port on your system and is not currently mounted. Also, it cannot have any important information stored on it since the entire drive will be destructively re-formatted by the following procedure.
Next, open GParted, which is located in the System submenu of the main menu. The location of the main menu is dependent on which desktop profile you're using. It's in the lower left for the Centrych profile and in the upper left for the Xcentrych profile. Both should have an icon that's the same as the icon in the blue title bar at the top of this web page.
Once opened, it should look similar to the following:
The drop-down widget in the upper-right shows
Then, as shown above, select Create Partition Table... from the Device menu. The following dialog will open:
Click on the Apply button to create a new, empty partition on the flash drive. When the operation completes you will be returned to the main window.
Right-click on the unallocated drive entry and select the New menu item as shown above.
Change File system: to fat32.
While Label: is set to "MULTIBOOT" as shown above, that is just a suggestion. You can enter any label you prefer.
Once complete, click on the Add button to return to the main window.
When you click the green arrow as indicated above to apply the pending create partition operation, the following confirmation dialog will open:
Click on the Apply button to start the create and format partition operation. The following dialog will open while that process is in progress:
The blue bar will slide back and forth while the operation is in progress, Once complete it change to the following:
Click on the Close button to exit the dialog and return to the main window:
Please return to the previous page, your USB flash drive is now ready to continue with the rest of the multi-boot procedure.