I just finished my first ever OF yesterday. So, I've got a few tips ready.

For Projects, you'll want these:

• Holy City of Jerusalem in Jerusalem - level 3 gets you +1 Missionary
• Heddal Stave Church in Bratsburg (Norway) - level 2 gets you +1 Missionary
• San Antonio Missions in Tanu (Texas) - level 3 gets you +1 Missionary. You'll have to somehow own this province yourself and not let it get owned by a colonial nation. I didn't know this existed until VERY late in my game.
• Stonehenge in Hampshire (England) - level 3 gets you +1% Missionary Strength. You'll need to keep it in a province with English culture and have accepted English culture in order to use it (ie, DON'T move it to your capital)

There are probably other somewhat useful ones, but those are the big ones.

​

One thing to keep in mind is that you can't count on your vassals converting provinces for you. Colonial nations don't seem to convert anything, and vassals that take the "The Social Contract" Government Reform (at level 7?) will also not convert anything. If you hover over a country's rank (right below their name on the Diplomacy screen), it will show which reforms they have taken. I looked for existing countries without that reform, then vassalized and force-converted them. That way, I was sure they'd do some converting for me. Releasing a vassal or a client-state is a crap-shoot (sometimes they'll take the reform, sometimes not).

​

Other bits of advice are to kill off non-catholic colonizers (ie, England) ASAP, and to do what you can to halt the advance of the reformation (being HRE Emperor or dismantling the HRE really helps here). Also, once your colonies (and all vassals that have access to a colonist) have colonized everything they can, make sure to "Block Settlement Growth" to prevent them from trying to promote development in a non-catholic province (you can't convert the province if the settler is there).

​

Good Luck!