When you've been doing this for awhile, and you both feel comfortable with where it is going, you might be ready for something else, and this something else can be a pretty big step. Don't even think of suggesting it until you have firmly established a baseline of sitting with her and occasionally cuddling.
Okay. Remember how you sometimes brush your fingers through her hair when you're cuddling? Well, women love to have their heads scratched. It does feel good - why do you think so many men fall asleep in the barber chair? So, do that more. Run your fingers through her hair, rubbing her scalp. Use your fingernails and lightly make long scratching rubs on her scalp. Don't make it the focus. Do it as if it's just something you're doing absentmindedly. If you're in the right moment and you have a firm foundation, she's liable to start purring like a cat.
If she says, "What the hell are you doing?", then you know you acted too soon. Immediately stop, act as if nothing happened, and back up a step. Don't be concerned if she acts a little cold with you for awhile. It's no big deal. It's not a show stopper. You just got a little ahead of yourself, and you need to spend more time establishing the basis of touching affection in your relationship. I should mention that recently I commented that she should teach you how to scratch her head. She said something like, "Oh, he'd never do that," and then kind of muttered, "Besides, that would be kind of creepy." I think that means a lot. First, it means that she never expects you to show her that kind of affection. But second, it means that she considers it a level of intimacy that might not be appropriate. So I'm telling you, if you get to where you can do this with her, it will mean a lot on several different levels.
If she does give you indications she likes it, then ask how you can do it better. You might even tell her that you want to be the best damned head scratcher in the world for her, and you can't do it if she doesn't give you instructions. What does this do for you? You're emphasizing that you want, more than anything, to please her. You're also indicating that you want her to teach you. There may come a time in the future where she's teaching you how to do other things that please her.
Make this a common pattern. Every time you sit with her and she's cuddling into you, rub her head. Make sure she likes it. Make sure she is comfortable enough with you that she wants you to do it. This will take time. Remember, this is not a horse race. This is a long journey, and you have all the time in the world. Spend time at it. Set a baseline.
Now the best position for a good head scratch is for her to rest her head in your lap so you can use both hands on her. She might suggest that, but chances are she will not. That is a pretty big step, isn't it? Having your Mom's head resting on your lap? (Stop. Close your eyes, and visualize that for a minute. Your Mother's head in your lap.) So, she might likely be very shy about bringing it up. If she doesn't suggest it, then you can prompt it by stopping rubbing her head - and you want to do this when she is clearly enjoying it in the moment. Tell her that the position cramps your hand, grab a pillow and put it on your lap, and she will get the idea. I think she'll plop her head right down on the pillow and you can give her the best head rub she's ever had.
If she does not go for it, or shows resistance, then that's okay. You'll get there. Don't make a big deal of it, but in the future just shorten the head scratching. Every time you quit - right when she's in the middle of enjoying it - say your hand is cramping. And occasionally put that pillow in your lap and suggest she just lay down for a minute so you can do it right. She'll get there.