How to to started relationship?Watch
2 get to know them enough for each of you to make an informed decision on whether you should spend more time together.
3 Spend more time together. Do more physically intimate things: touching, then kissing and cuddling then sex. When you've had sex you've started a sexual relationship.
4 Keep spending time with them until it's gotten to the stage where one or both of you would be better off without the other.
There are ways that you can maximise the amount of the people that you start talking to. These will require courage.
There are ways that you can maximise your chances of coming over as a person that they should have a relationship with.
There are ways you can speak and behave that will maximise your chances of retaining them as your sexual partner.
All of which would take multiple videos or articles or books to cover in any sort of depth.