In the simplest terms, "walking by the spirit" is a term which expresses conducting ones life by the abilities and power imparted by holy spirit. Holy spirit is obtained when anyone becomes "born from above"; acknowledging Jesus Christ as Lord (boss, director, single authority). This acknowledgment recognizes that the living God, his father, raised him from the dead. Anyone who arrives at that point in their life have done so in response to the action of holy spirit in their life. That is all that is required to receive the gift of holy spirit with all its benefits and responsibilities.
The God and father of Jesus Christ is holy and He is spirit. Receiving the gift of holy spirit is receiving the presence and life of God. Scripture refers to as this as "Christ in You" because it imparts to you (and all the brothers and sisters in Christ) the same ability and authority as Christ has; it is available because of the accomplishments of Jesus Christ.
Scripture also expresses that there is an "old self" of the flesh and a "new self" of the spirit. Building upon the concept of "Christ in You", each believer has the "mind of Christ". With this power from God, the old physical self has been transformed into a new spiritual self with a new nature, new abilities and a new identity. Because of the presence of God in you, you have the ability to act as an agent of God in the same manner as Jesus Christ did.
Just as a baby has all the abilities to function and grow in the physical realm as a human being, you have all the abilities to function and grow spiritually, with the presence of God in you! Just as a baby physically grows, develops and matures, so you can spiritually grow, develop and mature.
If this is so, then why don't people automatically manifest the power of God with more effectiveness? Why don't people manifest the deliverance that such power imparts? There are three aspects to the answer.
First, the systems of the world (especially religion) have taught and deceived people to ignore the spiritual side of life. People have been deceived into stunting their growth. The systems of this world (especially religion) work with deception and error to impart spiritual bondage, blindness and slumber to all. The systems of this world (especially religion) are designed to detect whenever a person is walking by the spirit of God. When such a person is detected, pernicious spiritual pressure is applied to shut down the Godly spiritual energizing in that person, or to limit their effectiveness.
Second, people are taught and deceived to ignore the effects of holy spirit. It often requires spiritual perception to be able to perceive the operation of the spirit of God. Although people are walking by the spirit and the spirit of God is moving in and around us, all are deceived to be blind to this reality. The spirits of blindness and slumber, which infect the religious realms, work hard to keep all from seeing anything Godly.
Third, the systems of this world (especially religion) teach and deceive with a multitude of errors regarding what it is to walk by the spirit and what to expect. People are deceived with errors regarding godliness, prosperity, deliverance and the power of God.
The single most effective deception which stunts spiritual growth and limits spiritual effectiveness is sin consciousness and self condemnation. Self condemnation is engendered and fostered by religious legalism. Religion and religious legalism is antithetical to walking by the spirit of God.
Regardless of working of the systems of this world and the spiritual forces arrayed against the agents of God, walking by the spirit continues to be an available reality to all those who desire to function and live through Christ for the living God.
In our culture, we are taught that life is like cooking food; we are taught that success is as simple as finding and applying the correct recipe. This concept is a life-wasting simplistic deception which ignores the spiritual side of life. Walking by the spirit is not learned or performed by recipe. Don't waste time or energy in the unfruitful search for recipes!
No book can teach how to snow ski; a book cannot teach a baby to walk. No book can teach how to walk by the spirit. What a book (or epistle or blog) can do is to introduce spiritual ideas and concepts which are needed in order to walk by the spirit. This is similar to what the epistle of I John does.
As an agent of God, who walks by the spirit, your purpose is to represent God and bring forth his will in your life, his will in the lives of those around you and his will in the physical realm. The agent of God manifests God's love in order to love others in the manner in which God desires for them to be loved. The wisdom and power of God is required to enable his agent to accomplish his will.