Most people, when “stuck” in top Half Guard, are looking to make their way to pinning the opponent down to the floor. Of course, there’s nothing wrong with that; on the contrary, your primary goal should be to press your opponent’s back down to the mats. However, the person you’re competing with knows that this is your goal as well – and the chances are that they’ll actively resist whenever you attempt to do this.
So where does that leave you? Yes – it gives you a chance to go for a Kneebar! Here’s one way of setting it up from this position, as explained by Nick “Chewy” Albin.

KNEEBAR FROM TOP HALF GUARD

To begin with this Kneebar setup, you should establish wrist control and push on the opponent’s shoulder in an attempt to level their back down to the mats. However, when you notice them giving resistance, you should let go of your pressure and let them go into you a bit.
As they’re going into you, you’re going to shift your weight so that you can step over their top leg.

Once you do this, Chewy emphasizes that you ought to grab the opponent’s heel as fast as possible – as it will give you the greatest possible leverage and opportunity to finish the submission.
From there, you can do one of two things. First, you can position the opponent’s toe next to your ear (the one closest to the mats), twist their heel a bit and extend your hips to get the tap. Or you could feed it under your far-sided armpit and get a bigger extension on the knee that way.

Check out this Kneebar setup, as well as a defense to it, on the video below: