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.
Additionally, scratch the inside of your upper lip with your nails before partaking in putting your fingers inside anyone. This is a good indication of how it might feel inside a vagina or butt.
You should be keeping your shit trimmed and clean anyway but sometimes you may have a little sharp edge you may have missed and this is a good indication of that before you go causing other people discomfort.
It is just the considerate thing to do :D
If you do have sharp edges after trimming them, a hack I’ve used is to rub your nails on a set of jeans
Reinventing the nail file
For some of us, jeans are more often close at hand than a nail file…