In plain words
Surrendering all actions to Me, with your mind centered in Ātma, free from expectation and possessiveness, and without inner fever, fight.
Word for word
Laying down all actions in Me with a mind on the self, having become free from expectation and free of 'mine', fight, with your fever gone.
The commentators
Bhagavadgītābhāṣyam
कथं पुनः कर्मण्यधिकृतेन अज्ञेन मुमुक्षुणा कर्म कर्तव्यमिति, उच्यते —
मयि वासुदेवे परमेश्वरे सर्वज्ञे सर्वात्मनि सर्वाणि कर्माणि संन्यस्य निक्षिप्य अध्यात्मचेतसा विवेकबुद्ध्या ‘अहं कर्ता ईश्वराय भृत्यवत् करोमि’ इत्यनया बुद्ध्या । किञ्च, निराशीः त्यक्ताशीः निर्ममः ममभावश्च निर्गतः यस्य तव स त्वं निर्ममो भूत्वा युध्यस्व विगतज्वरः विगतसन्तापः विगतशोकः सन्नित्यर्थः ॥ ३० ॥
In plain words
But then how should action be done by someone who is qualified to act, who does not yet know the Self, and who wants liberation? To this it is said —
Lay every single action down onto me — onto Vāsudeva, the Supreme Lord, the one who knows everything, the Self within all beings. Let go of it with a mind turned toward the Self, holding this one thought: 'Yes, I act — but only the way a servant acts, carrying out what the Lord wills, not what I want.' That is the conviction to hold. And more than that: let go of hope, let go of wanting a particular outcome. Let go of 'mine' too — let that whole feeling simply leave you. Once it has, fight, Arjuna — fight as someone free of that feeling. Fight as someone whose fever has broken, whose anguish has lifted, whose grief is simply gone.
Close to the Sanskrit
But how, again, is action to be performed by one qualified for action, who is ignorant and desirous of liberation? — thus (the question); it is said —
In me — in Vāsudeva, in the Supreme Lord, the omniscient one, the Self of all — having renounced, having laid down, all actions, with a mind fixed on the Self, that is, with the discriminating intellect, [thinking] 'I am the doer, but I act for the Lord, like a servant' — by this very conviction. Moreover: without hope, one whose desire has been abandoned; without the sense of 'mine' — the feeling of 'mine' having departed from whom, your — that you, having become one without the sense of 'mine', fight! — being one from whom fever, anguish, and grief have departed. That is the meaning.