Kneel down (so she is not too high) and pick her up. Support her by her hind legs and bottom with one hand, and her front legs with the other. Hold her loosely so that she does not feel constrained and can jump away if she wants to. It is important that you do not try to restrain or pet her.
Once she has got used to that, sit on a chair and place her on your lap. Again, let her jump off immediately if she does not like it. Do not try to restrain or pet her.
Persevere with this, and she will eventually realise that it is safe to be held by you and sit on your lap. Once she learns to relax, you will be able to pet her.
My cat prefers to sit on my lap, rather than be held, so you could start with sitting her on your lap instead of holding her.
Food is the best conditioner of all, so you could try tempting her onto your lap by hand-feeding her with her favourite treats (my cat loves ham). Again, you must allow her to leave as soon as she wants to.
Finally, some cats don't like to be held at all, and it is possible that you may just have to live with that. Remember that cats don't have owners, they have staff.
P.S. It took my cat a good three years to feel entirely comfortable with being held by me, and is only now allowing my mother to stroke him while he is in my arms. He still will not let Mum pick him up. You need to be very patient with some cats.