Start with the reason. A pilgrimage made in thanksgiving, for a sick relative, or to mark a milestone points to different places than one made to understand the faith from its roots. The existing page on choosing a pilgrimage when you do not know where to go walks through that decision; the short version is that Rome suits a first pilgrimage for most people, the Holy Land suits those drawn to the Gospels, and Lourdes, Fátima, and Medjugorje suit Marian devotion and a settled stay in one place.
Then decide between a group and independent travel. A priest-led group handles daily Mass, church bookings, and the Wednesday papal audience ticket; independent pilgrims can do all of this themselves but must plan it, and the existing page on guided versus independent travel compares the two. Parish groups are the most common entry point and usually form eight to twelve months ahead.
Booking requires a U.S. passport. The State Department lists mailing, processing, and return time that together can run two months or more, and some European countries ask for six months' validity beyond the travel dates, so check the expiration date before paying a deposit. Enroll in the State Department's STEP program and buy travel medical insurance; Medicare does not cover care abroad.
Spiritual preparation is simpler than people expect: confession before departure, a short daily prayer, one book about the place, and a written intention. The existing page on preparing spiritually goes further. On the pilgrimage itself, attend what is scheduled, rest when the group rests, and leave gaps unfilled.