I think my own stories make that pretty obvious.
What I like in a story is more extreme than what I want to experience for real, but both have hard boundaries. No burns, cuts, or blood bruises. Nothing that causes any lasting damage or pain that lingers more than a half a day, tops. Nothing that makes a person scream or shout in pain. In fiction, tears are okay, but in reality, not. Everything follows the standard SSC rule--safe, sane, consensual.
The best way to write pain is the same as the best way to write sex--and pretty much everything else. Get deep inside the point of view (POV) of the character, so that you know not just what's happening to their body but how it feels, what they're thinking, what's happening emotionally, whether it triggers any memories or hopes or fears or disgust, like that. You want your reader to share the POV character's mind, not just their body.
Getting into deep POV is huge. Examples just off the cuff:
Shallow POV:
Jeffrey decided he would spank Susan when she got home from her job as a librarian. "You better not!" she said. But he grabbed her by the waist and put her across his lap and spanked her until she cried. Afterward she was mad even though he kissed where he'd spanked.
What's wrong with this? Well, we don't share what either of the two characters is thinking beyond Jeffrey's decision. It's fairly uninvolving. It also tells instead of shows, which is a whole other weakness.
Deeper POV
Susan was late coming home from the library; the check-out desk was swamped because Marnie was taking one of her endless bathroom breaks.
"There you are," Jeffrey said. "You're so late you get a spanking."
What the hell? "You better not." Susan was in no mood for Jeffrey's kink. She was tired and wanted a martini, not a spanking.
As usual, Jeffrey didn't care. He grabbed her by the waist. Fighting him was pointless; he was a big man who wouldn't stop until he got her across his lap.
Moments later she lay over legs thick as tree trunks. As the first swats landed on her skirt, Susan vowed she wouldn't give him the satisfaction of protesting or trying to get away. She lay limp, accepting, even though near the end she cried not in pain but from frustration.
Of course Jeffrey wanted sex afterward. He kissed her tender buttocks. She endured it, just as she had the spanking. Tomorrow, she told herself. Tomorrow, once he left for work, she'd call in sick so she could pack up her things. Marnie would let her stay over for a few weeks.
Longer, yes, but Susan has some personality, a lot going on in her head while the physical scene is playing out. Of course it doesn't has to be that the POV character doesn't like it. I could have made her love it so much she pushes Jeffrey to go further than he wants to. But either way, she's far more than a cardboard cutout existing only to be spanked.