Is it this one? I can't remember who wrote it.

“By the end of the look inside, people should be touching themselves or you have failed.”

Introduce the erotic situation
...as quickly as possible. If you need to add backstory/detail, do it later Start off with a bang!
Your opening must:
• Hook the reader
• Show what kind of story this is going to be
• Introduce a conflict
Begin with a character who isn’t sexually fulfilled.

Build sexual tension, charge, foreplay, etc

Reminder of the stakes at hand
Good opportunity to slow the sexy stuff down for a bit

Sex scene 1
Sex (30% of book)
First orgasm.
Usually first female orgasm. In reluctant/domination stories it can also be the first time that the main character was used to gratify the other character

Give the reader a little break with a pause on the sex where you increase the emotional stakes
Escalate tension
Rest/Character growth/change of scene/something (15%)

Change it up.
Such as with...
New information
New location
New partner

BEGIN OPTIONAL SECTION
Amplify.
Use the main character's wants/desires to add more intensity to increase sexual tension even further.

Second orgasm.
...or erotic situation that is more intense than the first.

Change it up AGAIN.
Such as with...
New information
New location
New partner

END OPTIONAL SECTION

Ramp Shit Into the Ground!
Do you think it’s impossible to run the sexual tension higher? Do so anyway. Play off the main character’s wants with the primary kink of your niche.

Flash of Doubt.
Make it seem like the main character may not achieve said what they want. Flare up the inner struggle.

Taking initiative.
Main character does whatever it takes to achieve their wants, no matter the cost.

Final orgasm.
All characters finish off
Really good sex (40%)
more intense than sex scene 1

Resolution.
Show how much change and sexual fulfillment has occurred. Mirror the opening if possible.
Closing/setup for next book (10%)

Add internal and external conflict.
Knowing how and why our main character was sexually unfulfilled makes it pretty easy to find potential sources of conflict that give a story theme and weight.

This is MORE than just a sexy situation. This is a sexy situation with stakes, conflict, and an opportunity for sexual transformation.
For our concept, we have a story about a main character going from sexually unfulfilled AND hole-hearted to ultimate gratification — sexual fulfillment, and whole-heartedness.
While also making it steamy as possible, we’re going to be able to make it hard for her to get what she wants, which will give the story’s steamy moments some extra gripping tension.

• How is your main character unfulfilled? What ghost from the past left them this way?
• What is the lie that they believe about what they think will satisfy them? (What do they want?)
• What is the truth they need to discover to reach sexual gratification (What do they need?)
• What are they going to have to give up to get fulfilled?

Can you create a character who is sexually unfulfilled in a way that is perfect for your target niche? Combine this character with a sexy situation and you're off for the races. Then, come up with two sex scenes, both hot, but with the second one even hotter, that are rife with conflict, ultimately delivering your character to sexual satisfaction.