Hey mate,
broad question indeed.
Before you can build sexual attraction, you need to be sexual yourself.
then comes the question: what is being sexual? How is one sexual?
Hard to answer, but let me try. Other people can correct me or add their own thoughts.
Being sexual is loving sex, is being open about sex and also being open about loving sex. I would say that is one part of it. An other part is also being passionate about sex. you want to be good at it.
Tension is build by blocking a urge with an obstacle. This obstacle can be literally an obstacle, but it can also be something abstract, like an idea.
Example of litrally an obstacle that builds tension: Suppose you are hungry as hell but the cooky closet is closed by a key. The closetdoor is literally an obstacle building tension, because you are hungry, the cookies are within your reach, but the closet is in the way. I think you can follow this, right?
Example of a more abstact obstacle: Suppose you need to take a shit real bad. Like, really really bad, like in: it feels like something is already making its way through your circular rear muscle. But you are walking on the street. Social etiquette says that you cannot take a shit in the middle of the street. Social etiquette is an abstract obstacle, creating a lot of tension.
When you want to build sexual tension, you want to create an urge for sex and then create an abstract obstacle to block the urge. The obstacle can be something really stupid, but it just has to be there. Building sexual tension is actually part of flirting.
Examples: you and your date are in a fancy restaurant. Here goes the conversation:
you: you know what, if it wasnt for all these nice people sitting here, i would really fancy doing some naughty stuff to you...
her: (giggle) really? Hmmm, i dont know if would want that (winks with her eye)
you: well, ofcourse you wouldnt, in the middle of the restaurant... Even though i would kiss you from your neck to your shoulders to your.... belly button.... But ofcourse, i cant here in the middle of the restaurant..
her: hmmm you might be right. It would be very bad. But worse would even be if i start kissing you back, first on your lips, and then on your... cheek... but ofcourse, in a restaurant like this, it would be very bad
(you see, she plays along, lets pump up the temperature). You: hmmm, would you now... i dont think i could let you do that you know... what is if the waiter catches us on the act? What if was to get under this table, open up your pants and give a couple of licks down there while you are ordering drinks? i mean, we couldnt do that right? what if he sees me while ordering?
etc etc etc
do you understand a bit? The conversation doesnt need to make sense at all, you just start slowly with building tension. By that i mean, dont make your first flirtuous sentence like: i would fuck you hard in the ass doggy style if it wasnt for all these people here.... If she doesnt know you, this is not really flirting, its plain weird. So you need to start softly and turn up the heat along the way. That is building tension.
cheers and good luck!