Galation 5: 16-23
"16: This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh. 17: For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would. 18: But if ye be led of the Spirit, ye are not under the law. 19: Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, 20: Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, 21: Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God. 22: But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, 23: Meekness, temperance: against such there is no law
Basically walking in the spirit is walking by the spirit's revelations, the fruits of the spirit oppose to the works of the flesh.
The way of the flesh is easy to follow, you just do what comes natural and many walk in it but the way of the Spirit is more difficult because it takes dicipline and a denying of your flesh.
ex: You want to fornicate or commit adultry and you know that's wrong and of the flesh and if you give in you are walking in the flesh, but if you fight the urge and use temperance (fruit of spirit) and say no, I'm not going to do it and wait on God through longsuffering(fruit of spirit) then you're dening your flesh, therefore walking in the spirit. You are obeying the spirit rather than the flesh.
This is not an overnight process it is a growing process, but the more you walk with the Lord the more manifestion of the fruit of the spirit you should began to see manifesting in your life.
The more you allow the manifestation of the fruit of the spirit in your life the more you'll look like Christ.
JMHO
Be blessed