The front headlock in wrestling is easily one of the most uncomfortable positions to be caught in, and it's an incredibly effective technique, with both offensive and defensive applications. What follows is the fundamentals of the position, with special emphasis placed on posture and body positioning, and the initial "catch" or entry.
The first thing to understand about the front headlock is that it fits in with an overall wrestling strategy on the feet. First, determine if you have access to the person's lower body, at which case you can start thinking about single and double leg takedowns, among other lower body attacks. Clearly, if your partner has a tall posture, and you are shorter than they are, you won't be entering into a front headlock unless they shoot in on you. The moral of the story is to look for the more vulnerable area, and to attack that.
While it borders on a truism, you absolutely have to start with the basics, and you have to maintain them. Don't enter into the front headlock while leaving your legs unguarded, open to your partner's double or single leg attack. Instead, keep your hand posture correct. Use your left hand as a defensive block, but also as a fundamental part of your entry into the front headlock. With your right hand at about chest level, rake your partner's head forward, utilizing your forearm for this, aiming to guide their head into your left forearm (when the movement is complete). Finish the initial set up by keeping your right elbow pointed downward.
Note that your bottom hand, around your partner's chin, and your top hand (over their neck) make a sort of cradle for your partner's head. Next, pinch your forearms on the side of your partner's neck, with the ultimate goal being to keep the "cradle" space smaller than your partner's head. Keep their head in the center of your body, not allowing your partner to sidestep and take you down (or pass your guard!). Ultimately, your chest should create downward pressure on the back of your partner's neck, making good posture virtually impossible for them.
It's very important that you keep your elbows in tight to your body, and away from where your partner can grip them (and shoot or duck under as a result). If your elbows are out, you likely don't have very good control over this potential outcome.
You won't need to use this grip every time, but if you need it, it's there. If you can't reach all the way across to your partner's chin, you can grip their jaw bone instead and use this as a handle. Keeping the elbow in at the jaw is crucial here, or else your partner may be able to close the gap and take you down. Instead, use your arm as a post, keeping them at bay.