...as if he were his own enemy lying in ambush

Epictetus, Enchiridion, Chapter 48:

The mark and attitude of the ordinary man: never look for help or harm from yourself, only from outsiders. The mark and attitude of the philosopher: look for help and harm exclusively from yourself.

And the signs of a person making progress: he never criticizes, praises, blames or points the finger, or represents himself as knowing or amounting to anything. If he experiences frustration or disappointment, he points the finger at himself. If he is praised, he is more amused than elated. And if he is criticized, he won't bother to respond. He walks around as if he were an invalid, careful not to move a healing limb before it's at full strength. He has expunged all desire, and made the things that are contrary to nature and in his control the sole target of his aversion. Impulse he only uses with detachment. He does not care if he comes across as stupid or naive. In a word, he keeps an eye on himself as if he were his own enemy lying in ambush.

Advice to those just starting out: keep it simple

Epictetus, Enchiridion, Chapter 2:

Direct aversion only towards things that are under your control and alien to your nature, and you will not fall victim to any of the things that you dislike...as for desire, suspend it completely for now...restrict yourself to choice and refusal, and exercise them carefully with discipline and detachment.

Epictetus, Enchiridion, Chapter 8:

Don't hope that events will turn out the way you want, welcome events in whichever way they happen: this is the path to peace.

Advice to those just starting out: rehearse

Epictetus, Enchiridion, Chapter 4:

Whenever planning an action, mentally rehearse what the plan entails... 'I want [to do x], but at the same time I want to keep my will aligned with nature.' Do it with every act. That way if something occurs to spoil your [plans], you will have ready the thought, 'Well this was not my only intention, I also meant to keep my will in line with nature - which is impossible if I go all to pieces whenever anything bad happens.'

Here's the sort of formulation that gives Stoicism a bad name...

Epictetus, Discourses, Book IV, 4:

It would be lovelier still if you could secure happiness, free of emotion, poised and dependent on no one except yourself.

Here the view of emotion is overly simplistic: emotion equals weakness. The role of emotion in motivation (for virtues like courage), attention, or even the proper functioning of memory, is completely unrecognized.

If you observe rather than are observed...

Epictetus, Discourses, Book IV, 4:

If you keep yourself calm, poised and dignified, if you observe rather than are observed, if you don't envy people with greater success, don't let externals disconcert you - if you do all this, what more do you need?

Stop being jerked like a puppet

Marcus Aurelius, Meditations, Book Seven, 29:

Discard your misperceptions.
Stop being jerked like a puppet.
Limit yourself to the present.

Marcus Aurelius, Meditations, Book Four, 2:

No random actions, none not based on underlying principles.

How to act

Marcus Aurelius, Meditations, Book Three, 5:

Never under compulsion, out of selfishness, without forethought, with misgivings.
Don't gussy up your thoughts.
No surplus words or unnecessary actions.

Life is warfare and a journey far from home...

Marcus Aurelius, Meditations, Book Seven, 61:

Not a dancer but a wrestler: waiting, poised and dug in, for sudden assaults.

On the nexus of virtue and focused attention

Marcus Aurelius, Meditations, 5:

Concentrate every minute like a Roman - like a man - on doing what's in front of you with precise and genuine seriousness, tenderly, willingly, with justice. And on freeing yourself from other distractions. Yes, you can - if you do everything as if it were the last thing you were doing in your life, and stop being aimless, stop letting your emotions override what your mind tells you, stop being hypocritical, self-centered, irritable.