The easiest and hardest way would probably be to contact the person who uploaded it & ask them if they saved all the chapters somewhere, which they probably did, & e-mail them to you. Otherwise, probably shady download sites it is.
If you browse chapters while logged in and you load in the entire chapter (as opposed to browsing page by page), you can just save (ctrl+s) the page to your hard drive. When you browse for it, you'll have all the files loaded by the website, which will contain the images as well. Just delete what you don't need and volia.
They *do* have the same names the original uploader gave them, so you might need to sort them yourself if they aren't already.
Alternatively, if you bought the chapters, you can do it on Lezhin instead. Some panels will be broken, but at least you'll have the best quality available.
As for legality... Well, we're on Mangago. Go figure.
Eeeek! :3 thank you so muchhhh!! <333
As the title implies, if there is a way that is safe and not illegal, please let me know >.< I'd like to treasure this manwha and it's story forever in my life.... <333