This question has been asked of me quite a few times.
The simple answer, often the answer that those posing the question don’t want to hear, is: You don’t. The greatest oath you can make is a vow not to bind yourself to him.
Regardless of whether you’re approaching Lucifer and Luciferianism through atheism, theism, or agnosticism, there are a couple different reasons why being oathbound would prove to be a problem.
The first would be the goal of apotheosis. If you are seeking to become your own god, why would you make a vow to follow another god for the rest of your days? Even if you are only seeing him as a means of support for your own growth, the goal is to not need that support.
Additionally, depending on your level of faithfulness to keeping that oath, it may later prove to be a self-imposed restriction on your own free will—a restriction that Lucifer as a god would not want in the first place.
You should ask yourself, what is the point of making an oath to a god of change, a god who finds beauty in the ephemeral? What use would a sworn oath be to a god who recoils from absolute fealty and worship directed towards him?
Even if you did make an oath, he would find a way to make you go back on your word, along with the faith that fueled its initiation. As Adversary and Destroyer, it is what he does best. He will make you question all that you thought you loved about him, all that you held dear, because a resolve to remain steadfast and loyal to him as you believe him to be is a resolve to never embrace change or doubt, a resolve to be stagnant in your faith.
Take it from someone who once made such a vow—the only acceptable binding oath made to him is a broken one.