RCIA, or the rite of Christian initiation for adults, is the traditional method for people to become Roman Catholic. The process includes regular meetings, spiritual reflection and participation in a number of church rites, according to an article on RCIA by the Archdiocese of Philadelphia. It can take up to a year. Under special circumstances, you can become Catholic without participating in RCIA.
The process of becoming Catholic without RCIA is similar to the traditional method. You will have a series of meetings, most likely with your parish priest or another faith leader such as a deacon. They will likely be one on one rather than group meetings with other faith candidates. The focus of the meetings will depend upon whether you were baptized or confirmed in the church. Faith preparation is a key component, but it can be guided by your spiritual needs and circumstances. Unlike traditional RCIA, the content of the personal meetings will be up to you and your faith leader, as is the time required before you can formally join the church. At the end of the process, you will be presented to your parish and welcomed as a Catholic in a special rite generally presided over by a bishop. While generally this is done on Holy Saturday during the Easter Vigil, it can be done at other times if needed. After you join the church, there is an additional period of reflection and instruction that most new Catholics do so they are comfortable in their new community. This can be done publicly or privately, as your situation dictates.
Certain situations are more likely to warrant a personal faith journey to Catholicism as opposed to traditional RCIA. A parish priest can evaluate those who seek to join the church to judge if an exception to RCIA is needed. If personal issues prevent you from fully participating in a group RCIA, ask your parish priest about an alternative. For example, a person who has experienced abuse may be unwilling or unable to share personal information in a group, instead preferring a one-on-one spiritual exploration that is more self-guided. Mentally disabled people usually cannot fully participate in RCIA. Many parishes will, if asked, structure special sessions to distill the Catholic faith into easily understandable concepts. The emphasis is less on cognitive analysis and more on the emotional acceptance of faith. If you are a non-Catholic marrying a Catholic, you may want to join your future spouse's faith community more quickly than RCIA allows. If you are motivated, your parish priest will likely meet regularly and share faith precepts so you can become Catholic. While the Catholic Church doesn't prevent someone from marrying outside the faith, you may prefer to share the same religion as your spouse. Work or family commitments are among the other reasons a person might not be able to attend regular RCIA sessions. For example, if you are a single parent who juggles a demanding job, you may not be available for each RCIA meeting. In that case, your priest can arrange for private meetings at more convenient times.