Romans 8:9-11 // Assurance

But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his. 10 And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness. 11 But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you. (Romans 8:9-11)

Here is the answer to the question of how we may live after the Spirit. If we ask God to give us the Holy Spirit in the name of Jesus he will live in you and me. (Luke 11:13; John 14:14)
So you and I who are converted and have the Holy Spirit are not living after the flesh but after the Spirit. Christ is working on us to change us by his Spirit, we may not be sinless yet, but he is working on it. If we are not living in submission to the Holy Spirit's working in our lives we are not a true Christian, we are not saved.
If on the other hand we are living in submission to God, our life has been laid down. We have died to our reputation among our friends, we have died to our dreams of worldly greatness, we have died to our plans for our happiness. But more literally, like every human on Earth, we are living under a death sentence because we have sinned.
Notice that Paul does not say: "our sinful spirit is dead" or "our innocence is dead". Paul is not teaching original sin here, he says the body is dead because of sin and he doesn't stop there. He goes on to say  but the Spirit is life because of righteousness because the Holy Spirit is able to give us life because of the righteousness of Jesus Christ.
If the Holy Spirit was able and willing to raise Jesus from the dead, he will also raise those of us who follow Jesus unto death. Some would say, that even now the Holy Spirit is what gives us life, but I don't see evidence for that here or elsewhere. Someone is welcome to share if they do.
We have God's promises now, he promised to give us the Holy Spirit, and he promises that when we receive the Holy Spirit we are living after the spirit. Do not waste this gift, keep following God's leading and do not let the Devil turn your attention to your weaknesses all the time.