It’s not really easy, and in some ways its not really useful, to list off requirements, because at once there are no requirements for salvation and far too many to ever list.
Let me build this from the ground up.
In Catholic teaching, ultimately what matters is if a person loves God - and because loving God requires loving neighbor, to love neighbor too. However, since this is included in loving God, we can just say to love God.
And this hits on the key. To love God is both very simple and very complicated. It’s simple in itself, but there are also many things that go in to loving God. Loving God is really one requirement that breaks down into many. If we really love God, we will approach Him with faith, and obey Him. If we obey Him, we will fulfill His ordinances, such as baptism. If we obey Him, we will keep His commandments. If we love Him, we will love our neighbor, which means we will do lots of different things for them, etc.
Now ultimately, we get this Love only through Jesus Christ. Apart from Him, it is impossible for a person to love God. You can’t give what you don’t have, so we have to have God’s Love given to us. God does this through Grace. When a person receives God’s Love through Grace, they are united to His mystical body. He left us the Church on earth which is mystical his body. Anytime then, that a person accepts the Grace to love God, they are united to the Church. Every Protestant that loves God is united to the Church, even if they don’t know it and even if they would consciously deny any affiliation with the Catholic Church. Those that love God are united to the Church.
Now, if a person loves God, he is going to go where God asks him to. If a person then has the knowledge that the Catholic Church is Christ’s Church and refuses to visibly be united, this is an act of not loving God, and so that person by that act is failing to fulfill the one real requirement of loving God, and so will not be saved unless he repents. If a person ought to know the Catholic Church is Christ’s Church but in some way refuses to admit it to themselves or perhaps to consider it, that too would be an act of not loving God. On the other hand, if a Protestant, or anyone else for that matter, honestly through no fault of their own did not know, and honestly believed they were doing what God asked of them, then they would be loving God, thus united to the Church and through it be saved. The person would be loving God, because they would be doing the very best they knew how to do what He asked of them.
(Btw, baptism is required, even though for those who don’t know it’s necessary they can still be saved without water baptism. I left this out because it will cloud the discussion, but if you are interested in this, just ask.)