Don’t just keep the finger straight as you go. Not only do you miss major pleasure centers, but you might actually hit the cervix, which may result in pain or discomfort. Curving the finger upwards means it achieves optimal friction over the G-spot and, is you reach deep enough, slides easily into the anterior fornix to stimulate the A-spot. If you don’t know, that’s the pocket just above the cervix that the penis would optimally slide into when doing front-every PIV (missionary, cowgirl, etc).
This is assuming the person is on their back. The curve should be towards the anterior (front) side of their body.
Recommend applying firm pressure and starting with a “come here” motion, kinda like scratching an itch, but with your fingertip. You could also vary this up with firm circular motions on the spot.
In my limited experience, the g-spot is a spongy area on the interior surface of the pelvic bone. That’s your target… I can’t speak for fingering the a-spot