Hilchos Maasei Hakarbanos 5:7

Sin offerings that are eaten require four placements of blood, on the upper half of the four corners of the outer altar. Regarding this, Leviticus 4:25 says, “On the corners of the altar.” The kohein brings the blood in the bowl to the altar, where he dips his right index finger into it. He puts his thumb on one side of the altar and his middle finger on the other, and effects atonement. [Having his thumb on one side and his middle finger on the other positions his middle finger, which was dipped in blood, on the actual corner.] He lowers his hand from the top down until all the blood has been rubbed off his finger, repeating this procedure on each corner. If he put the blood within a cubit (about 18”) of the corner on either side, atonement is still effected.

Hilchos Maasei Hakarbanos 5:8

The kohein must dip his finger in the blood at each corner. When he finishes placing the blood on a corner, he must clean his finger on the rim of the bowl. He then dips it again because the blood already on his finger is unfit to put on a different corner.