As a self-publishing author, you sometimes have way more time than money. For some, there is also a level of creative satisfaction in mastering a new skill.
Book covers and dust jackets are a case in point.
Once you become profitable or have the funds, it is better to have a professional designer create them for you. They will do a better job and you free up some valuable writing time.
Until then, doing it yourself may be ok if you have a design eye and the right tools.
Photoshop or InDesign would be great but most authors don’t have those. Most people however do have Microsoft Word and this tool can definitely do the job.
This great article shows you how:
If all else fails eg you don’t have Word or an eye for design there’s always someone on fiverr who can help you!