By his spirit - The word spirit here is either synonymous with wisdom, referring to the wisdom by which God made the heavens; or with breath - meaning, that he did it by his own command. There is no evidence that Job refers to the Third Person of the Trinity - the Holy Spirit - as being especially engaged in the work of creation. The word spirit is often used to denote one's self; and the meaning here is, that God had done it. This was one of the exhibitions of his power and skill.
He hath garnished the heavens - He has formed the stars which constitute so beautiful an ornament of the heavens.
His hand hath formed the crooked serpent - Or, rather, the fleeing serpent - ברח נחשׁ nāchâsh bârı̂ach; see the notes at Isaiah 27:1. There can be no doubt that Job refers here to one of the constellations, which it seems was then known as the serpent or dragon. The practice of forming pictures of the heavens, with a somewhat fanciful resemblance to animals, was one of the most early devices of astronomy, and was evidently known in the time of Job; compare the notes at Job 9:9. The object was, probably, to aid the memory; and though the arrangement is entirely arbitrary, and the resemblance wholly fanciful, yet it is still continued in the works of astronomy, as a convenient help to the memory, and as aiding in the description of the heavenly bodies. This is probably the same constellation which is described by Virgil, in language that strikingly resembles that here uscd by Job:
Maximus hic flexu sinuoso elabitur anguis
Circum, perque duas in morem fluminis Arctos,
Arctos oceani metuentes sequore tingi.
Geor. i. 244.
Around our pole the spiry Dragon glides,
And, like a winding stream, the Bears divides;
The less and greater, who by Fate's decree
Abhor to die beneath the Southern sea.
Dryden
The figure of the Serpent, or "the Dragon," is still one of the constellations of the heavens, and there can be little doubt that it is the same that is referred to in this ancient book. On the celestial globes it is drawn between the Ursa Major and Cepheus, and is made to embrace the pole of the ecliptic in its convolutions. The head of the monster is under the foot of Hercules; then there is a coil tending eastwardly about 17 degrees north of Lyra; then he winds northwardly about 14 degrees to the second coil, where he reaches almost to the girdle of Cepheus; then he loops down and makes a third coil somewhat in the shape of the letter "U," about 15 degrees below the first; and then he holds a westerly course for about 13 degrees, and passes between the head of the Greater and the tail of the Lesser Bear. The constellation has 80 stars; including four of the second magnitude, seven of the third, and twelve of the fourth.
The origin of the name given to this constellation, and the reason why it was given, are unknown. It has been supposed that the Dragon in his tortuous windings is symbolic of the oblique course of the stars, and particularly that it was designed to designate the motion of the pole of the equator around the pole of the ecliptic, produced by the precession of the equinoxes. It may be doubted, however, whether this is not a refinement; for the giving of a name for such a cause must have been based on knowledge much in advance of that which was possessed when this name was given. Mythologists say, that Draco was the watchful dragon which guarded the golden apples in the garden of the Hesperides, near Mount Atlas, in Africa, and which was slain by Hercules. Juno is said to have taken the Dragon up to heaven, and to have made a constellation of him, as a reward for his faithful services. The origin of the division of the stars into constellations is now unknown.
It has been known from the earliest times, and is found in all nations; and it is remarkable that about the same mode of division is observed, and about the same names are given to the constellations. This would seem to indicate that they had a common origin; and probably that is to be found in Chaldea, Arabia, or Egypt. Sir Isaac Newton regards Egypt as the parental point; Sir William Jones, Chaldea; Mr. Montucla, Arabia. There is probably no book earlier than this of Job, and the mention here of the names of the constellations is probably the first on record. If so, then the first intimation that we have of them was from Arabia; but still it may have been that Job derived his views from Egypt or Chaldea. The sense in the passage before us is, that the greatness and glory of God are seen by forming the beautiful and the glorious constellations that adorn the sky.

By his Spirit he hath garnished the heavens - See the observations below.

By his spirit he hath garnished the heavens; his hand hath formed the crooked (l) serpent.
(l) Which is a figure of stars shaped like a serpent, because of the crookedness.

By his spirit he hath garnished the heavens,.... The visible heavens, with the sun, moon, and stars, with which they are studded and bespangled, and look exceeding beautiful; and the invisible heavens, with angels, the morning stars, and glorified saints, who especially in the resurrection morn will shine not only like stars, but as the sun in the firmament of heaven; and the church, which is the heaven below, is garnished with Gospel ministers, adorned with the gifts and graces of the spirit of God:
his hand hath formed the crooked serpent; because Job in the preceding clause has respect to the heavens and the ornament of them, this has led many to think that some constellation in the heavens is meant by the crooked serpent, either the galaxy, or milky way, as Ben Gersom and others; or the dragon star, as some in Aben Ezra (c): but rather Job descends again to the sea, and concludes with taking notice of the wonderful work of God, the leviathan, with which God himself concludes his discourse with him in the close of this book, which is called as here the crooked or "bar serpent", Isaiah 27:1; and so the Targum understands it,
"his hand hath created leviathan, which is like unto a biting serpent.''
Some understand it of the crocodile, and the epithet agrees with it, whether it be rendered a "bar serpent", as some (d); that is, straight, stretched out, long, as a bar, the reverse of our version; or "fleeing" (e), as others; the crocodile being, as Pliny (f) says, terrible to those that flee from it, but flees from those that pursue it. Jarchi interprets it of Pharaoh, or leviathan, both an emblem of Satan, the old serpent, the devil, who is God's creature, made by him as a creature, though not made a serpent, or a devil, by him, which was of himself. Some have observed the trinity of persons in these words, and who doubtless were concerned in the creation of all things; here is "Jehovah", of whom the whole context is; and "his Spirit", who, as he moved upon the face of the waters at the first creation, is here said to beautify and adorn the heavens; "and his hand"; his Son, the power and wisdom of God, by whom he made all things.
(c) So Dickinson. Physic. Vet. & Vera, c. 9. sect. 23. p. 137. (d) "serpentem vectem", Pagninus, Cocceius; "oblongum instar vectis", Schmidt; "oblongum", Junius & Tremellius, Piscator; "longa trabe rectior". Vide Metamorph. l. 3. Fab. 1. ver. 78. (e) "Fugacem", Montanus, Vatablus; "fugiens", Codurcus. (f) Nat. Hist. l. 8. c. 25.

UMBREIT less simply, "By His breath He maketh the heavens to revive": namely, His wind dissipates the clouds, which obscured the shining stars. And so the next clause in contrast, "His hand doth strangle," that is, obscures the north constellation, the dragon. Pagan astronomy typified the flood trying to destroy the ark by the dragon constellation, about to devour the moon in its eclipsed crescent-shape like a boat (Job 3:8, Margin). But better as English Version (Psalm 33:6).
crooked--implying the oblique course, of the stars, or the ecliptic. "Fleeing" or "swift" [UMBREIT] (Isaiah 27:1). This particular constellation is made to represent the splendor of all the stars.