“prepared.” The Hebrew verb is in the hiphel aspect, causative, thus “prepared” seemed the correct meaning here (cp. “prepared,” Darby; Douay-Rheims; Geneva Bible; KJV; Rotherham. Cp. “made ready” BBE).
“the heavens.” The noun is always plural, so it could also be translated “heaven.”
“the horizon above the face of the deep.” Or, “inscribed a circle.” The “deep” here refers to the oceans on the face of the earth, and God inscribed a circle upon them, which we see as the horizon.