Ask. My bf has mentioned it a few times and while there's nothing particularly off putting about it I'm a bit confused about the logistics and it's making me anxious.

My brain: Is this happening on the sofa? Am I one end and he's the other? How do I start this spontaneously? My feet are never in that area. How do I physically do that? I don't have much foot dexterity. I'm going to have the silliest concentration face going on if I attempt this. It's not going to be sexy in the slightest and the chances of managing it to any decent degree that would lead to a happy ending is practically zero.

That's just how my annoying brain processed this simple request. Maybe your gf is normal and would just bypass all of that but, just in case- I'd suggest asking in the moment and gently bringing her feet there and help guide her, giving praise as you go.