You need to be sincere that you just want to fuck. There's no need to verbalize this. You can express it non-verbally.
One technique is scarcity. You don't hook up with a girl everyday or once a week. Hook up once every two weeks or once a month if you haven't fucked her yet. Just make sure that you give her the best possible fun that she can have when she's with you. This basically means that you shouldn't be reactive to any drama from her or from your environment.
When you've already fucked her, keep the fuck marathons and hangouts to once a week only. Do NOT bring her to movies or fancy restaurants.
Remember: You just want to fuck her. You shouldn't treat her like a girlfriend. Be sincere. Do not hold her hand or claw your arm around her shoulder when you two are hanging out. Do not mix up your libido with cheesy actions that you see in movies.
When you two are together, keep the topics to sex only. Do not talk about goals, your job, your problems, your family, or your friends.
Another technique is implication. When she's hanging out in your place, make sure that you have some supplies of girlie things like pink toothbrushes, some disposable panties, face powder, girlie lotions, make up and so on. After fucking a girl for the first time, take out your supplies and offer it to her.
If she asks why are you keeping those things, say "I have close friends who sleepover." Then cut. Do not explain anything or elaborate furthermore. If she tries to sleepover again, it's on. She'll know.
There are more tricks in the bag but I'll give the stage to other PUAs who specialize in multiple long term sexual relationships.

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Approach. Open. Escalate. Isolate
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