en_tn/lev/08/15.md

horns of the altar

This refers to the corners of the altar. They are shaped like the horns of an ox. See how you translated this in Leviticus 4:7.

set it apart for God

"set the altar apart for God"

in order to make atonement for it

Here "atonement" means to make the altar fit for use in serving God. Alternate translation: "in order to make it a suitable place for burning sacrifices for sin"