The most natural way is just to be more vocal during sex. Make it clear how much you enjoy each smack. Tell him you’ve been a naughty girl, and deserve to be spanked. After each spank, say “Harder!” and keep saying it after every other spank until he establishes the force you desire.

I recommend reassuring him by saying something along the lines of “Ooo yes! Like that! But harder!” This is actually very important, as a guy’s number one worry during a spanking, especially when getting started, is not coming across as a complete brute

That’s why you need to give him the signals that what he’s doing is what you want. We men have many talents, but mind-reading is not one of them, especially when we have erections. That’s why the submissive young woman who says she’s been naughty and deserves a spanking is such a hot male fantasy. It is explicit permission to proceed – for your lover to transition from being your protector to your punisher.

If you need inspiration, take a look at this scene from The Brass Teapot:

Be like her, be direct, tell him it’s ok, and be playful. Tell him how naughty you’ve been. Tell him you rubbed your clit in your office toilet thinking of his stiff cock. Or you went for a walk without any panties on.

Give him your consent. And he will give you the spanking you desire…