First things first, every cat intro is different! Don't use this video or any cat intro guide as a hard and fast rule book, you'll need to adapt to your own specific circumstances.
1. Go slow, it's much harder to fix a negative interaction than it is to prevent them.
2. Hissing is not aggressive, please don't punish this behavior.
3. Shoot for tolerance, not friendship. You just need to them to coexist without drama!
Alastor was about a year old when he came home, Foxy is 14y. That's essentially having a teenager move into a retired woman's home and wreak havoc, so give them both a bit of grace! It took me ~1 week to feel comfortable leaving them unsupervised, and ~2-3 weeks before we started seeing any playing.
Hope it's helpful, let me know what you think in the comments!