impromptu new kitten, one male cat already at home, tips please

hello everyone! I apologise if I’ve done anything wrong here, I’m more of a commenter than a poster myself I’ve had my baby boy for 3 years now, light of my life <3 he’s an indoor kitty, very shy to newbies but never aggressive, always been pretty chill. However, my mum messaged me earlier and a woman has kittens she desperately needed to rehome else she was going to have to “let them loose”, which I cannot bear the thought of. So now, I have a female kitten arriving tonight. This was very unplanned. My boy is utterly spoilt, so I have all the tools and whatnot for a kitten - however, I would love some help on introducing them and how to make it less stressful for them both! My boy has always chirped at other cats through the window, but when I had my friend’s cat over for an hour once (avoiding the landlord!) he would growl when he could see the cat and just sat outside the door most the time.

I know keep them separate, different litters, don’t feed near one another to start etc, but I’m open to all and every tip!

Thank you so much <3