First, disqualification and recusal are very different. Disqualification is mandatory and even jurisdictional, and it can be raised at any stage of the litigation. It is non-waivable. If the judge made rulings in a case where he or she was subject to disqualification, the orders are void.
The grounds for disqualification come from the Texas Constitution (Art. V, Sec. 11), but they are restated in Tex. R. Civ. P. 18b. The judge is disqualified if he or a member of his prior law firm served as counsel in the case; if he has an economic interest in the case, directly or as a fiduciary; or if any of the parties are related to him by the third degree of relationship. This means that if any party, either by blood or adoption, has a relationship to the judge of father, mother, child, grandchild, or great-grandchild, aunt or uncle who is a sibling of one of the judge's parents, or a nephew or niece who is a child of one of the judge's siblings, then the judge is disqualified. Usually if a judge is disqualified, they get out of the case on their own without the necessity of motion.
Recusal is more subject to discretion and is waivable. The grounds can sometimes be the subject of differing opinions. They include where a judge's impartiality might reasonably be questioned; if he or she has a personal bias or prejudice about the subject matter of the case or knowledge of evidence material to the case; he or a lawyer with whom he practiced has been a material witness to it; his spouse or minor child has a potential financial interest in the case that is material; or if he or his spouse or anyone within the third degree of relationship is a lawyer in the case.
The recusal rule is phrased in mandatory terms; so, some of the grounds for recusal can be effectively mandatory if there is no doubt about their applicability. For instance, if the judge's daughter is a lawyer in the case, he must recuse. Some of the other grounds, though, are phrased in a manner where opinions could differ.
It has been held that receipt of campaign contributions is not a ground for recusal.
The procedure for recusal is set out in Te. R. Civ P. 18a. The motion must be filed at least ten days before the date set for the hearing. Once it is filed, the judge subject to the motion may not take any further action in the case until the recusal motion is resolved. If the judge declines to recuse voluntarily, he must send the motion materials to the presiding judge of the administrative judicial district, who will hear the motion or appoint another judge to hear it. The judge who is the subject of the motion cannot decide the motion. If the motion is denied, and it is found that the motion was brought solely for purpose of delay and without sufficient cause, the judge hearing the motion may impose any sanction authorized by Tex. R. Civ. P. 215(2)(b).
If you would like to read more about this subject, it is covered in the first volume of my treatise that I co-author with former Justice Scott Brister, Texas Pretrial Practice (James Publ. 2011), Sections 16.162-16.260; that treatise is available in most Texas law libraries or could be bought at the James Publishing website.