Even if you do decide to follow these instructions, I suggest you read the LiveUsbPendrivePersistent wiki page. Reading Wiki pages is a great way to learn and you'll know more about what you're doing - rather than just following steps blindly.
Normally, I'd suggest that you use the Startup Disk Creator in Ubuntu to do this - it has a really simple option:
that allows you to add MB of space to keep stored in reserved extra space. However, there is sometimes a bug with installing the bootloader:
at least for me. You also mentioned that you'd had issues with Startup Disk Creator created USB drives being so.
So, I'd suggest you use Unetbootin. It's not quite as polished, but it does the exact same job and has the exact same store in reserve functionality. It's what I use when creating LiveUSBs and has yet to fail me.
It's in the default repositories, so you can install it via
sudo apt-get update && sudo apt-get install unetbootin.
Once it's installed, open up the dash (Super key - aka Windows key) and open up unetbootin - you'll have to search for it. It'll require the sudo password, just a heads up. You'd need sudo access just to install the app, so that's no problem.
Once unetbootin starts, you'll want to change the radio button from selecting
Distribution (default choice) to selecting
Diskimage and then click the ... button. You can then select the ISO and it will show up in the text box.
Once you've selected the ISO you want, make sure the type is set to
USB drive and that it's targeting the correct USB drive. If you've got USB backup media or other USB sticks attached to your computer, you may want to consider disconnecting them for a bit - just to be safe and sure you don't accidentally overwrite some important data.
Now, to add the persistence of data, simply change the
Space used to preserve files across reboots (Ubuntu only) value. This should create a file-system on the USB stick, along with the ISO contents, that will store your information across reboots!
Note, that if you use Unetbootin to create the LiveUSB, you'll have to specify the space in MB (MegaBytes) not GB (Gigabytes). A simple way to convert this is just punch into a calculator
numberOfGigabytes * 1024. That will provide you with the correct amount of space in MB terms (as there are 1024 MB in a GB.)