In plain words
In a clean place, let one establish a firm seat for oneself, neither too high nor too low, covered with cloth, skin, and kusa grass.
Word for word
In a clean place having set up for oneself a firm seat, not too high, not too low, with cloth, skin and kuśa grass in layers,
शुचौ देशे प्रतिष्ठाप्य स्थिरमासनमात्मनःनात्युच्छ्रितं नातिनीचं चैलाजिनकुशोत्तरम्
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
अथेदानीं योगं युञ्जतः आसनाहारविहारादीनां योगसाधनत्वेन नियमो वक्तव्यः, प्राप्तयोगस्य लक्षणं तत्फलादि च, इत्यत आरभ्यते । तत्र आसनमेव तावत् प्रथममुच्यते —
शुचौ शुद्धे विविक्ते स्वभावतः संस्कारतो वा, देशे स्थाने प्रतिष्ठाप्य स्थिरम् अचलम् आत्मनः आसनं नात्युच्छ्रितं नातीव उच्छ्रितं न अपि अतिनीचम् , तच्च चैलाजिनकुशोत्तरं चैलम् अजिनं कुशाश्च उत्तरे यस्मिन् आसने तत् आसनं चैलाजिनकुशोत्तरम् । पाठक्रमाद्विपरीतः अत्र क्रमः चैलादीनाम् ॥ ११ ॥
In plain words
Now, for someone actually taking up yoga, the rules have to be laid down — how to sit, what to eat, how to move about, and the rest of it — because these are the means of yoga. And along with that, the marks of one who has arrived at yoga, and what comes of it. That is what begins here. And of all these, posture is spoken of first —
Set your own seat in a place that is pure—clean and undisturbed, whether by its own nature or by ritual cleansing—steady, unmoving, neither too high nor too low. Layer it with cloth over deerskin over kuśa-grass. And notice: the order you actually lay them down in is the reverse of the order the verse names them in.
Close to the Sanskrit
Now, at this point, for one practising yoga, the regulation of posture, food, movement and the rest, in their capacity as means of yoga, is to be stated, and the defining marks of one who has attained yoga, and its fruit and so on — therefore this is begun. Therein, posture alone is first stated, to begin with —
Having set up, in a pure—clean, secluded—place or spot, by nature or by ritual purification, a steady, unmoving seat of his own, not too high—not excessively raised—nor even too low; and that seat is ‘having cloth, deerskin, kuśa-grass on top’—that is, cloth, deerskin, and kuśa-grass are laid on top, on which seat that seat is ‘cloth-deerskin-kuśa on top.’ Here, the order of cloth and the rest, in actual arrangement, is the reverse of the order given in the text.