Is there a proper way for Christians to have sex?
The proper place for sex is within a marriage. "Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for fornicators and adulterers God will judge" (Hebrews 13:4).
The proper ways to have sex are those that show love and respect for each other. "Nevertheless, each individual among you also is to love his own wife even as himself, and the wife must see to it that she respects her husband" (Ephesians 5:33). If you are asking about certain positions or techniques during sexual intercourse, those are not specified by God.