I'm not going to lie I kind of stumbled into getting her to hook on, I didn't have a game plan or anything - it just sort of happened.
Often does 'just happen', if you have a reasonable 'feel'. My pony isn't the only one by far who is 'hooked on' to my kids without them even knowing what it means. Often doesn't even(especially?) work when people are trying really hard & following a bunch of instructions!
I teach a horse to yield to direct(fingertip or such) and indirect(finger or whip pointed, bodylanguage) pressure in a variety of ways. They learn to move away from the pressure when it's 'on' but not to keep trying to 'escape' when it's off. 'Free lunging' etc is just doing that at a distance with no strings attached.
So... very basically, if a horse is facing me & I want to 'send' her, I'd point out in the direction I want & direct a bit of pressure at her inside(what I want to be the inside) shoulder to get her to turn away. Then I direct a bit of pressure out behind her if I want her to go/speed up. To slow down, stop or backup, the cue is pressure out in front & the cue to face me is pressure directed at the hind quarters to get them to turn away.
It's easier to start out - especially if you don't use any positive reinforcement - in a small paddock, large pen or on line, to have more control, to block or make 'wrong' behaviour less likely & reinforce 'right' behaviour more. Once it's going well like that, you can 'up the ante' & try it without strings & in different environments.