Stoicism is a practical branch of philosophy (seems like an oxymoron, but it isn't) focused on finding a better way to live one's everyday life.
Don't Let Emotions Rule #
Emotions are important for understanding our reactions to events and learning more about ourselves, but they shouldn't pull us down the wrong path either.
Treat sudden emotions and unwanted thoughts like a passing breeze - you acknowledge them without letting them pull your thoughts down a bad path, and eventually they pass by.
That's not to say we shouldn't base any actions on emotion, since sometimes that's exactly the case. But knowing to act on important, larger emotions than short-term ones we'd regret after is a big difference. Acting on love that's lasted for years is different than acting on sudden rage or despair.
Take Only What You Need #
Avoid too much attachment and longing for physical things, since it simply fuels a desire for more and more things which can never be satisfied. It's a neverending spiral that takes away our self-control, and therefore our freedom.
So be satisfied with as little as you can. Wring all the enjoyment and pleasure from as few possessions as possible.
Avoid buying things due to impulse or "shiny object" syndrome. Put off any purchase for a few days, and if you're still thinking of the item and wanting to get it, then you can consider it more.
Accepting a Lack of Control #
There's very few things in the world we have control over. Virtually everything outside of ourselves we can't control, and many things about our own selves (parts of our health and personality) we can't control either.
Fighting against this by trying to control everything ultimately makes one bitter, angry, and upset over living in an "unjust world." So the solution is to focus on what we can control - our reactions to what we can't control.
This goes back to not letting emotions and unwanted thoughts rule - we can't always control our emotions and thoughts, but we can control our reactions by letting them pass by without affecting our actions.
It also means finding the good side in what we can't control. If we're stuck in a traffic jam, getting angry at the cars is pointless since there's little we can do to make the traffic go faster. So one would accept that without anger and focus on what they can control - how they feel and make use of the situation. For example, the person could use the extra time to meditate on their life or listen to an audiobook or podcast they've been meaning to catch up on.
Nothing is Permanent #
All items, people, and problems we deal with are temporary and will ultimately fade one day. This includes ourselves, since ultimately we will die.
This is painful to think of, but keeping it in mind helps us keep calm and a sense of perspective. Knowing all bad things are temporary keeps us from overreacting to them. Knowing all good things are temporary also keeps us from getting complacent or relying on them to establish our sense of self. Our sense of self shouldn't come from physical objects anyway, it should come from self-awareness of who we are.
Death is the end of all feeling, good and bad, so death itself is neither good or bad. It is simply an unavoidable, neutral end we have no control over - therefore, becoming too stressed or anxious over it is pointless. But it can still help death act as a motivator, as knowing your days as numbered makes you treasure each moment more and find more meaning in them.
Don't Avoid Misfortune #
Like death, misfortune is inevitable and impossible to predict. So we should prepare ourselves for tragedy and discomfort ahead of time. A simple way is imagining worst-case scenarios and how they would affect your life's path, seeing how you would adjust. This helps reassure your mind and plan ahead for how you'd adjust your path should something horrible happen.
A good, everyday way to do this is voluntary discomfort, or doing small things on purpose that make you less comfortable. This gets you used to less comfortable conditions, so it takes less to make you happy and relaxed. A classic example of this is taking colder showers.
The simplest way is to be more grateful for the simpler, smaller things in life. The more joy you can get from small things, the harder it'll be for tragedy to take it away.