@Bondppq , the video basically follows the description in post #7
Here is a more detailed view which follows that video shown in post #8.
The video shows the disk utility on the system being accessed to try and resize the partitions, but when the user tries to resize the disk, it can't be done because the disk is not unmounted, which is shown in the error message on screen.
The user therefore has "inserted a medium" which is a usb live disk of "Linux Mint 19.1 Cinnamon 64-bit". Running the live disk will leave the computer's disk system unmounted in the first instance.
With the live disk inserted, the computer's disk system is shut down, and rebooted into the boot menu where the user chooses to boot the live disk which is on the usb that was inserted. The live disk is then booted up, and the user chooses the GParted program on the live disk.
After the resizing by GParted, and proper exit from the live disk, the system is rebooted with a reminder on screen to remove the live disk so that the machine will boot to its own internal disk, now with the new partitions.
Now that the resizing has created "free space" the user further partitions the free space using the computer's disk system which is possible because the free space is not mounted. The system's disk utility is used for this. I can't say what that utility is but it looks like it'll do the work that GParted does, if it's not GParted or a version or clone of it itself.
After creating 2 partitions, as an exercise, the user deletes one of them with the computer's disk system disk utility, evidently to show how partitions can be manipulated. This resizing is possible as long as the partitions are unmounted.
What is not shown is the creation of a filesystem on the new partition which must be applied so that the partition becomes usable for the system. The default filesystem is usually ext4 which can be applied with the mk2fs or mk2fs.ext4 commands.
To achieve the results you want, it's probably useful to check out how to use the GParted program at the link provided in post #7.