I thought about this thread a bit and want to address some of the claims/problems here.
1: the heat goes to the head and the head feels like it has a dent:
the heat going to the head is completely normal, but it is important how it feels, because when you meditate, it should always be comfortable. If you feel too excited during meditation, the medicine will not be born. One of the earliest major treatises on Nei Dan by Chen Tuan (Yin Zhen Ren Huan Dan Ge Zhu) says that you should focus first on the "Northern Sea" (lower dantian) which is the ultimate darkness (non action, non intention) and then the "Zheng Qi" (upright energy) will move to the head by the river cart (moving up the ren mai meridian). When the energy arrives at the head, you shouldn't force it, just let it be until it is ready to move on its own. The best thing to do is "bao yi" (contain as one. Your whole body seems to be one unbroken original qi). Trying to open the du mai, third eye, or any energy centre is the lower method of neidan, it is not as good as non intention.
2: the Dantian is not something you should use the intention to focus on:
the majority of Neidan documents suggest to focus on the Qi Xue point, which is just under the dantian. The problem is that people don't know how to focus and they think they need to place too much attention. You don't need to put much attention at all, just breathe to that area through the nose, soften the breath so you can't hear it, and allow your mind to settle in the lower abdomen until it seems like the mind disappears. The best way is that there is no you, no surroundings, and if you are successful, even no nothingness. That is how the real pre heaven three treasures are joined. If you focus on the breath, the intention, and the lower abdomen, that is post heaven and it is called "wai yao" or outer medicine. outer medicine is good for your health, but not good for spiritual purposes. It is better to direct the mind to genuine non action, in which the attention and breath are only used as an anchor, not as antagonist.
3: you should find a real teacher to learn in person:
I agree with this claim. The best thing to do is find a real teacher who can teach you face to face. This can be very hard to do though and many teachers who claim to teach neidan are actually teaching visualization or qi gong. Both visualization and qi gong can be good though and certainly you can get benefit from them, so don't discount them out of hand.
Studying from people online is possible and certainly I have personally taught a number of people, but out of the thirty or so people I've taught, only about three have been able to manifest what I would consider to be the basic level of Nei Dan practice (able to enter into non action and the pre heaven state). Most people would be better suited by studying Zen, or just counting their breaths, since those are much easier to learn and also have many good benefits. After a long time, I've come around to the conclusion that Nei Dan isn't for everyone and that anyone who is going to study it seriously must be very intelligent and very meticulous about practice and research.
Studying on your own is an even bigger risk and there are very few resources out there to tell you if what you are doing is right or wrong. If you want to meditate on your lower Dantian, you should just focus on that area, don't worry about the rest of your body. The most important thing is to maintain calm and stability. Don't mess around with the third eye or heart gate until you actually have a teacher to tell you what to do, because those practices can be very dangerous if you make mistakes.
I personally believe that meditating on the whole body as one piece is actually safer for most people than the lower dantian and it is much harder to make mistakes that way. If you are going to practice by yourself, then you need to research twice as seriously as someone who already has a teacher, so think carefully because the potential for bad results is very real and you probably don't want to go down that road.
I would also suggest that you check out other traditions which might have schools in your area. Neidan is one school of meditation among many and there are many other schools which can produce equal results, so be open minded and look around for a master level teacher. The best situation is you find a master level teacher in an art that works for you, the worst situation is that you find a bad teacher in the art you want to study.
Hopefully this has been somewhat useful.