Easy...

Don't. Fatigue, whether physical or mental (and usually the same), is a natural response to an overloaded system. Too much of a good thing can also cause stress!

Stress, overwork, too much hardy partying, are just a few of the things that can cause fatigue. Usually, fatigue is the result of several of these factors working against you at once. Medically speaking, the end result of stress is hard to diagnose. See adrenal fatigue as an example of a diagnosis controversy, for example.

(Image - "The Hunting of the Snark" by Lewis Carroll, 1st edition, 1876, Wikimedia Commons)

Sometimes, the fatigue doesn't kick in until the stress has been removed; that makes it much easier to deal with.
The common cure, however, is rest. Remove the stress and take it easy for a while, That is easier said than done, especially for people whose stress is also their livelihood. There are ways of coping with that situation, although as I'm resting now, I won't get into that specific discussion.

The best bet is to have a system that is resistant to stress. I am speaking of mental, emotional, and physical strength. Meditation, exercise, Stoicism, snuggle-bunnies, familial support, self-discipline, etc are all methods or devices that can be used to create strength in their respective areas. These are not the only ways, however, and each personality needs it's own approach.

Recognizing where you are in life and dealing with that is a paramount step. I am getting older and my vision acuity is dropping. The screen is a wee bit fuzzy to me as I type. The best way for me to deal with that is to get an eye exam. That would reduce a certain amount of stress in my own life.

So why do I say don't deal with stress?

Perhaps I should say instead, don't stress out about stress. There are a couple of Zen passages about this, although I don't recall them, and I'm not going to stress myself by looking for them. Perhaps I can paraphrase...
chillax, man

Take care of what you need to take care of, but don't stress yourself doing it. If looking up a mediation course at your the local adult education center is too stressful for you, ask someone else to do it for you, or just rest until you can do it yourself.

Fatigue can be dealt with by a weekend of sleeping in...IF the stress levels that caused it are minor compare to your strength levels. On the other hand, if you have been gritting it out for years, your recovery is going to taker a longer time.

But that first step is always going to be knowing yourself, and knowing how bad your stress level is.