If she likes having her hair pulled during sex, it’s easy enough if she’s on top, you just reach up and grab a handful. But most pull-my-hair types like being on the bottom and it’s harder to get a proper handful when she’s under you. You do need a proper handful because that spreads the pull across her scalp as a sense of pressure, rather than grabbing just a little hair and turning it into something painful… or a lost clump of hair.
The real issue is that the pillow gets in the way, so toss her pillow away. Then if she turns her head to one side, that allows you to get good access to most of her hair and get a good handful. Once you have a good hold, her head is fairly well stuck in place as some of your body weight will be resting on your elbow/forearm attached to the hand holding her hair.
This is a very firmly held position even compared to her being tied up with low end bondage equipment. That she could probably break out of if she wanted to, but you on top of her holding her head turned to the side leaves her with zero leverage. Unless you let her go, she’s… ahh… fucked. Which is the idea and the excitement, but also she may find it a little more than she expected. So a safeword may be in order. If you don’t have a safeword, consider “stop” or “let me go” or even “ow my hair!” acceptable attempts at using a safeword.
The upside is that she’s basically held stationary and you can’t (and shouldn’t) accidentally jerk on her head/neck and risk her injury. So it should be quite safe while being very firm.
And like anything else rough, this is something to talk about before you do it. Consent is important and besides… she’ll just be confused as hell if you throw her pillow off the bed without cluing her in.