Can a Pope declare a dogma, and how?
As the Baltimore Catechism states:
Q. 531. What is necessary that the Pope may speak infallibly or ex-cathedra?
A. That the Pope may speak infallibly, or ex-cathedra:
1. He must speak on a subject of faith or morals;
2. He must speak as the Vicar of Christ and to the whole Church;
3. He must indicate by certain words, such as, we define, we proclaim, etc., that he intends to speak infallibly.