The adage might be that you can't judge a book by its cover, but every author can tell you that is exactly what happens.
Most people in the industry, including other self-published authors, will give you the sound advice to hire someone to do it for you. For some, such as myself, there is absolutely no budget at all for it, let alone the $300 or so you'd need for a reasonably good cover for your work.
It can be done by yourself, but you need to be prepared for a lot of angst and frustration along the way, especially if you're committed to doing it all for free.
Here are some things you will need:
A photo editing/illustration program. The best free option is a program called GIMP. Shave your head bald before attempting to use it, so that you can spare yourself the pain of ripping out your hair as you try to understand the features. It does almost everything that the more expensive Photoshop will do. Photoshop can be used for free for 30 days on trial, but Adobe forbids "for-profit" usage during the trial. You've been warned. (By the way, you will NEVER come close to mastering Photoshop in 30 days or even reach the level of putting together an awesome cover. You've been double-warned.)
Access to royalty-free art. There are a ton of sites which offer this, such as Flickr, Pixabay and others. Pay close attention to the individual license on the picture. Some prohibit commercial use while allowing personal use (which means you can't use it for a book cover on a book you intend to sell). Others allow any use, but prohibit modification, meaning you can't change the colors or elements in the photo. In practical terms, that license is also useless. You could try your hand at taking the pictures you need, but you're going to find it far more challenging than you thought it might be. (Another warning, from someone who's tried.)
Access to royalty-free fonts. Just because you can download and install a font doesn't mean you can use it for your book cover. Google offers around 90 free fonts, about 5% of which are actually usable for a book cover. You can find other fonts using a Google search, there are some nice ones that you can use without the slightest hint of acknowledgement or payment. As with the other things you'll need, you shouldn't simply click on the license agreement without reading it. It's silly to risk find yourself sued (as the whole point is to save money) for lack of reading some of the most compelling literature around. You'll be smarter after you finish reading one, or at least, older.
An above average understanding of the "rules" of graphic design. I put rules in quotes because they are sort of suggestions, or possibly outrageous demands, or sound advice. Google will have lots to offer on the subject, nearly all of it confusing and inconsistent. The best advice I can give is to look at real book covers on actual books at a bookstore, and try to make your stuff look like their stuff. This may require you to erase the Comic Sans font from your computer so that you'll never, ever be tempted to use it for anything that you want to sell.
Patience and time. Count on spending a few weeks tinkering with your design. Just like your writing, you won't often get it right on the first try. Get a stress ball and a pillow to shout curses into while you learn the design software. Save a dollar a day while you're designing your own cover, and give up when you have enough saved to hire someone.
Research. Look at every book cover possible in your planned genre. Don't use some cute Martha Stewart font on a true crime book cover. Don't use a picture of a bloody knife on a children's story about cutting your finger. Know what is supposed to go on the book cover to match it to the genre.
I nearly forgot to mention tools that will build the cover for you, with minimal amount of effort. I'm not familiar with every tool, but I can speak to a couple of them:
Kindle Cover Creator: Honestly, I would say to not waste your time with this tool. Your 6-year-old niece is capable of putting together a better cover than this tool. The fonts are pretty limited, as are the templates. And you still largely need your own artwork.
Canva: Canva is a little more flexible and isn't limited to putting together book covers. You can do Twitter posts, logos, and tons of other things. The catch is that it's only mostly free. If you don't have the exact photo you want and need to look in the Canva library for it, it'll set you back $1. Doesn't sound like much and it isn't, but the better art is behind their paywall, which is a little pricier.
Best of luck no matter what you decide to do. I won't say that it isn't possible to do you own cover, as many self-published authors do just that. You should have some idea of what you're getting into before you try to do it yourself. The last thing you want to do is torpedo the potential sales of your well-written manuscript by putting a sub-par cover on it.