Well if you see on the right of the page next to the picture there will be an "install" button. You click that and it'll install to your profile. So, then you can open up to make a journal on the right of it there will be a "Style" part and under it will be the journal skins you have installed to your profile. You click on one of the skins when you have PM and it'll change the journal for you I hope that wasn't too confusing I'm bad at explaining things.
Well you have to make your own codes (by using guides that you can find all over the internet) and it should be easy to figure out how to put the code into your journal and this will teach you how to make your journal installable for other people to use; [link]
if you search up some tutorials and still have no idea how to use CSS Coding or it's a little too confusing (I've been playing with CSS Codes for years, so i know my way around) you can you this; [link] and it will generate a journal code for you