Instead of fingers you can also use a vibrator. Most girls really get excited by the strong vibrations and the constant rhythm.
Most girls who use a vibrator masturbating use it to stimulate the clitoris. So don’t insert it in her vagina but stroke the clitoris with the top. If she pulls back this might mean the stimulation is too intense. You can stimulate the clitoris indirectly by stroking the skin around the clitoris or stroke the inner lips. But it’s also possible to put some cloth in between the vibrator and her clitoris. Ask for directions.
Some girls like fingers or a dildo in their vagina, while you stimulate the clitoris with your other hand. So you might ask her about this.
Most girls will find it a nice surprise if you come up with a vibrator. But there are also girls who will think it is weird. So be careful if you introduce it, she might not like it.