How do you get an 8 year old cat who has never interacted with other cats to accept your new 3 month old kitten?

I myself, being the owner of many cats, can tell you this. Your cat may or may not accept your new kitten.

The only thing you can do is just introduce them slowly, don't force anything onto your older cat that it doesn't want.

The older cat will at first hiss, cuff, or growl at your new kitten, but in time he'll learn to tolerate the newcomer.

He just needs to learn that the kitten isn't there to take his place, make sure you're still giving your older cat the attention that he wants and show him that even though you have another cat, you still love him just as much.