That’s an important verse. Working out your salvation with fear and trembling begins by taking God seriously. It begins with having a high view of God, especially the holiness of God.
When people saw the holiness of God in the Bible, whether it was Isaiah in the temple, or John on the island of Patmos, or Peter on the Sea of Galilee, they had no trouble having the fear of God. Once they saw the holiness of God, it sobered them and dropped them to their knees.
So, in order to work out our salvation (“salvation” in Philippians 2:12 is sanctification) with fear and trembling, we need to have a greater and higher view of the holiness of God.