How to be separate . . . a holy people apart from the world? Individually, as it comes down to you....
It is not something you will do unless and until the Holy Spirit Himself begins transforming your character, otherwise it is just "whited sepulcher" business.
But you can pray for the Spirit to change you . . . so that you no longer hunger and crave worldly things and instead thirst for righteousness. Many, however, don't really want the change. They still love to partake, indulge, revel in ungodliness. So, the desire to move on from that is not strong, and not very sincere. Then one wonders why no real change is happening.
I know what it takes for me, right now, with the full assault against our minds going on in the culture, to stay "in the Spirit" . . . . And I think anyone who is cavalier about it makes a serious mistake, thinking you can be immersed in the culture, looking at it, imbibing it constantly, and yet remain "set apart" . . . .
What are you doing? How much effort, time, attention . . . are you giving to God and His Word and the work of His people? Just asking . . . . Not to compare or condemn, but just to share and encourage that we confirm and convict one another in this Walk . . . .
Anyone who thinks they can fill their mind with tempting images, words, teachings, feelings, allowing such inside their "temple" . . . and still expect the Holy Spirit to reside . . . is fooling themselves . . . .