partitioning

Is the computer a modern one using Secure Boot (UEFI)? If so, you might need to create a GPT partition table (instead of an MDOS one) on the USB device. You can use GParted to do that — beware! doing this will instantly erase all data on the drive, so be very sure that you've chosen your USB and not your computer's drive. But if your computer doesn't use Secure Boot, don't worry about this.

I don't know the purpose of the 4Gb FAT partition in the tutorial. Maybe it's spare space to be able to port files to a different computer.

So, I suggest the following.

If your computer uses Secure Boot, you'll need a small FAT32 partition. Size 500Mb is about right. Tell the installer to use this for its EFI System Partition. (If your computer doesn't use Secure Boot, skip this partition altogether.)

If you need Windows to access some files, create a partition for transferring the files. Instead of FAT32, I suggest NTFS. Make it only as large as you'll need for transferring files; it doesn't have to be 4Gb if you don't need that. Tell the installer not to use this partition. (If you don't need to share with Windows, skip this partition altogether.)

For the Linux system, use all of the remaining space and format it to ext4. Tell the installer to use this as the root partition.

Don't create a swap partition. Ubuntu 20.04 uses a swap file by default, though you probably want to avoid using swap on a USB stick — it will slow your system to a crawl.

EDIT: The tutorial doesn't mention that you must change the target bootloader installation (compare steps 6 and 7 (Phase 2) at the bottom of each image). Please remember to do this, otherwise your USB won't boot on a different machine, and you could have problems on your machine when the USB isn't plugged in.

EDIT: To change the target bootloader.