Repose your mind on Me (the Supreme) alone, and then establish the intelligence too in Me. From then on, you will certainly abide in Me (the Supreme), no doubt.
Krishna is explaining the content of true devotion, which is relevant to our inner personality alone. Mind and intelligence are the sole functionaries in devotion. World is a display of the Supreme, like waves of the sea.
By fixing the mind and intelligence in the Supreme, devotee gets fused into the Supreme. The whole life thereafter will be in the Supreme. Sense of difference or separation will vanish altogether. Oneness and sameness alone will be felt, expressed and extended.
Mind generally dwells in the world the senses perceive and interact with. Senses bring plurality, differences. On turning inward, getting tuned to the Self, the plight changes. All differences dissolve, and oneness prevails. Devotee begins to feel oneness with the Supreme and hence with all. The procedure is simple, and the outcome marvellous!