Pope Francis is a valid Pope. He is currently the only Roman Pontiff of the Catholic Church. Pope Benedict XVI validly resigned, and so he is no longer the Pope; that is why he is called “Pope emeritus”. Any Catholic who rejects Pope Francis as the valid and sole current Pope of the one true Church is in a state of formal schism, is automatically excommunicated, and may not receive any of the Sacraments (except Confession, once he is repentant). If you reject the Pope, you have separated yourself from formal communion with the Catholic Church.
For Catholics who accept Pope Francis as the valid Roman Pontiff, some disagreement is possible without heresy, schism, or other grave sin.
1. Personal Opinion
When Pope Francis expresses his personal opinion on a matter of faith or morals (or any other topic), and given that the Magisterium has no definitive teaching on the subject, the faithful Catholic is free to disagree.
Pope Benedict XVI wrote and published a book entitled, Jesus of Nazareth: from the Baptism in the Jordan to the Transfiguration. In the preface of that book, he writes:
“It goes without saying that this book is in no way an exercise of the magisterium, but is solely an expression of my personal search ‘for the face of the Lord’ (cf. Ps 27:8). Everyone is free, then, to contradict me. I would only ask my readers for that initial goodwill without which there can be no understanding.” 
Here Pope Benedict XVI gives us a good example to follow concerning the expression of theological opinions by the Bishops and the Pope. Such expressions, no matter how emphatically they may be phrased, are not an exercise of the Magisterium, and so are not binding on the faithful. All are free to disagree with the Pope in any personal opinion that he expresses: about Jesus, about matters of faith and morals, and certainly on other matters.
Caveat: In all likelihood, the opinion of the Pope on any matter pertaining to faith or morals is better than your opinion. The mere opinion of the Pope is fallible, but so are all your opinions.
2. Prudential Judgment
When Pope Francis issues a judgment of the prudential order, under his authority as Pope, but as a judgment not a teaching, the faithful Catholic is free to disagree.
Cardinal Joseph Ratzinger: “Not all moral issues have the same moral weight as abortion and euthanasia. For example, if a Catholic were to be at odds with the Holy Father on the application of capital punishment or on the decision to wage war, he would not for that reason be considered unworthy to present himself to receive Holy Communion. While the Church exhorts civil authorities to seek peace, not war, and to exercise discretion and mercy in imposing punishment on criminals, it may still be permissible to take up arms to repel an aggressor or to have recourse to capital punishment. There may be a legitimate diversity of opinion even among Catholics about waging war and applying the death penalty, but not however with regard to abortion and euthanasia.” 
Caveat: An official decision of the Pope under his temporal authority (the type of Church authority that exercises prudential judgment) may still be binding on you as a Catholic. So, for example, if the Pope changes the form of the Mass, you are free to think that the changes are imprudent, but you are not free to reject the Mass on that basis. Or if the Pope changes the rules for reception of Communion, you are free to argue that the changes are imprudent, but you are not free to reject the Mass or the Pope on that basis.
3. Non-infallible Teachings
Each and every teaching of the Magisterium falls into one of two categories: infallible or non-infallible. It is a common misunderstanding to think that all teachings of the Magisterium are entirely without error.
Pope John Paul II: “With respect to the non-infallible expressions of the authentic magisterium of the Church, these should be received with religious submission of mind and will.” 
The non-infallible teachings are reliable and have only a limited possibility of error, but they are NOT infallible. The errors possible in non-infallible teachings never reach to the extent of leading the faithful away from the path of salvation. But non-infallible teachings are non-irreformable. They are subject to a limited possibility of correction, improvement, and change.
“There exist in the Church a lawful freedom of inquiry and of thought and also general norms of licit dissent.” 
Non-infallible teachings are subject to a limited possibility of error and reform; therefore, they do not require the full assent of faith, but a lesser type of assent called the “religious submission of mind and will” . What this means is that you are generally required to believe the non-infallible teachings of the Church. But there is some room for faithful dissent, called “licit theological dissent” . To whatever extent a teaching might err, the faithful are free to disagree. God who is Truth never requires assent to false or erroneous ideas.
The non-infallible teachings of the Magisterium are full of truth. The number, type, and extent of the possible errors is quite limited.
An example of an error in a non-infallible teaching is found in the first edition of the Catechism of the Catholic Church, in the definition of lying, which erroneously stated that an assertion is only a lie if the person (to whom you are speaking) has a right to the truth. That claim was removed from the second edition.
Caveat: The basis for the disagreement must be Sacred Tradition, Sacred Scripture, or teachings of the Magisterium of greater authority. Most Catholics who reject a non-infallible teaching of the Magisterium have no legitimate basis for that rejection.
4. Infallible Teachings
The infallible teachings are free from all possibility of error due to the work of the Holy Spirit. The infallible teachings require your full assent with the virtue of faith; obstinate disagreement is the grave sin of heresy.
The infallible Magisterium is exercised in any of three ways:
a. Papal Infallibility
b. Conciliar Infallibility
c. the ordinary and universal Magisterium
To reject an infallible teaching of the Magisterium is material heresy. To do so knowingly and deliberately is the grave sin of formal heresy, which includes the penalty of automatic excommunication. If Pope Francis or any other valid Pope teaches something under any type of infallibility, you are required to give that teaching the full assent of faith. Otherwise, you commit heresy and formally separate yourself from the one true Church.
Caveat: Do not be fooled by those blind guides who claim that the Pope can commit heresy himself, and thereby lose his authority. Doctor of the Church Saint Robert Bellarmine held it to be “probable” that the Pope could never commit heresy personally, nor teach heresy in any way. He also held it to be “certain” that the Pope could never define a heresy as a teaching to be believed by the whole Church.
The foolish today claim that IF a Pope teaches heresy in a way that would seem to fall under infallibility, the teaching is nullified because the Pope fell into heresy. To the contrary, Saint Bellarmine believed that a Pope could NEVER teach heresy in a way that would seem to fall under infallibility.
Please take a look at this list of my books and booklets, and see if any topic interests you.
 Joseph Ratzinger, Benedict XVI, Jesus of Nazareth, p. xxiv.
 Cardinal Joseph Ratzinger, Worthiness to Receive Holy Communion, General Principles (sent by Cardinal Ratzinger to Cardinal McCarrick, Archbishop of Washington, D.C., and made public in July, 2004), n. 3;
 Address of Pope John Paul II to the Bishops from the United States on their ‘Ad Limina’ visit, 15 October 1988, n. 5.
 National Conference of Catholic Bishops (predecessor to the USCCB), Human Life in Our Day, “Norms of Licit Theological Dissent” n. 49 to 54; http://www.priestsforlife.org/magisterium/bishops/68-11-15humanlifeinourdaynccb.htm
 Second Vatican Council, Lumen Gentium, n. 25.