Note: Create your partitions from your empty space. You may need to resize your existing partition to do this. But don’t practice on your main drive.
This is a simple job, in that the steps are few, but it’s something that causes catastrophic data loss if you get it wrong.
I’d recommend buying a cheap second drive, doesn’t have to be big or even good. Partition it, mount it, make sure you can make the partitions automatically mount, teach yourself to copy data around, umount it and remount, make sure you got it right.
Just… these are all very simple things. I wouldn’t hesitate to repartition my own drives. But if you fuck it up you fuck it up good. Make sure you know the operations you’re taking first. Measure twice, cut once, all that jazz.