Numbers 8:11
And Aaron shall offer the Levites before the LORD for an offering of the children of Israel, that they may execute the service of the LORD. And Aaron shall offer the Levites before the LORD for an offering of the children of Israel, that they may execute the service of the LORD.
Numbers 8:11 belongs to the instructions given for the purification of the Levites. In that setting Aaron was to present the Levites before the Lord on behalf of the children of Israel. That presenting was done on behalf of Israel so that the Levites might perform the service of the Lord. By being thus presented to the Lord on behalf of Israel they were set apart to carry out the Lord's service.
The presenting of them was an offering of them to the Lord. As such an offering they were set apart to carry out the Lord's service in the sanctuary.