Introducing a new cat to my old cat.?

[ch3sh1r3_cat]

If anyone knows anything about successfully introducing a new cat to an older cat, I need some advice.

3 days ago I got a male cat (Phillip) from the SPCA, he's around 7 months old.
I have a resident female cat (Mikah) who's about 6 years old.

Both are fixed.

Here's my problem:

When I brought Phillip home, I let him instantly roam free around the house. He spent all day and night exploring and smelling everything. Mikah wasn't very happy as she sat up on high objects and just angrily stared and hissed/growled when he got too close.

Mikah is still not happy, hissing/growling even though its only been 3 days.

I googled "how to introduce a new cat to a resident cat" and I found info about how some people have had success in making a new cat become friends with an old cat, and I feel like I have made a mistake on letting Phillip roam free and be in Mikah's territory. I am kind of upset as I am feeling like I messed up and Mikah may never like Phillip. I don't really know what to do because theres apparently a huge process in introducing cats slowly and I already messed that up by letting Phil go anywhere he wants. Phil used Mikah's litter box and she used it even after he did.. is that a good sign?

I'm just stressing because my Mikah is really upset about this new cat, shes usually the nicest cat in the world and now she's showing a side I've never seen before, hissing and growling. I really want them to be friends but after reading forums about how other people got their cats to become friends, I'm worried that I messed that process up and they might never be friends.

Mikah had 2 male friends in the past, they were both older than her, and they all cleaned and sleeped with eachother, but since the 2 male cats passed away, Mikah has been the only cat for around 4 years so she got pretty comfortable with the idea of being the only cat.

I'm just wondering if anyone had any similar experiences on introducing new cats to an older cat, and what methoods worked and if I'm screwed since I let Phil just roam free in Mikahs areas right off the bat...