You will find out quickly that playing underhook half guard is made much more difficult when someone has a strong crossface or shoulder pressure across your neck. This kills your head movement, flattens you out, and if they also use their other arm well, renders your underhook useless. The problem is your opponent attaching their structure to you in a controlling way.
When you are able to detach their crossface and get your underhook, you can attach your structure to theirs in a way favorable to you! It sounds simple, but when some gorilla is driving their shoulder across your neck with full body weight behind it, it’s not so simple. The thing that makes all this much much easier is off-balancing.
The easiest starting point for off-balancing in half guard is simply to keep your legs triangled, squeeze your thighs together so you get a grip on your opponent, and pull your knees up to your chest. This is made easier with your arms lifting your opponent upwards at the same time, but for now the focus will be on the leg motion. What you will find in off-balancing this way is that it is demanding on your stomach and legs. They will get tired, you will get stronger, and over time this won’t be a limiting factor anymore. It is also helpful to do this while your opponent is still in motion. People get much harder to move when they settle down and lock into their base, so if you can off balance while they are still in motion, say, as they are adjusting their top half guard as you are transitioning into half guard, it is much easier to move them in a way that will help you.