I didn't make these, they were part of a modders resource I found a while back. I used the pack file manager 1.11 to make the pack file just to see whether it would work in NTW. And it did. This contains the flags for Austria (Austria-Hungary), Prussia (Germany), Russia, the Kingdom of Italy and the Ottomans. The naval signs are not included, nor are the agent icons. This is just a test after all.
You should only have to unzip the pack and place it in your data folder, but tell me if it doesn't work.
By the way, while I messed around in the packs, I noticed flags for american factions are there, both the US and indians.