How does a doctor know how far to put the needle into your back during a spinal tap?

Two things: knowledge and equipment.

Knowledge means a clear appreciation of spinal anatomy and the right place to put the needle to the skin, and the right position for the patient to be in, and the right angle. It means knowing that the needle punctures the skin, the adipose tissue, the supraspinous ligament, the interspinous ligament, the ligamentum flavum, the epidural space, the dura and the arachnoid. It also means knowing the surrounding structures in detail so that if you're off your line you know how to correct it.

Equipment means using the right needle. The correct type of needle has a "pencil point" like this:

This needle is slightly blunt, which provides excellent tactile feedback to the user. You can actually feel little pops and clicks as you advance the needle through all those structures I listed above. You get an especially large click as the needle crosses the flavum. An experienced operator will check the needle position at this point. No CSF: you would advance the needle for one more click: the dura. Then there is CSF, which is a clear fluid which drips slowly out of the hub of the needle and can be collected.

Some places still use a spinal needle with a sharp, cutting tip:

This quincke point is a weapon of mass destruction. Not only is it too sharp, so you can't feel where you are, it's more likely to cut a hole in the dura causing a dural puncture headache, and to damage the delicate nerve rootlets of the cauda equina.

A lumbar puncture is very much a learned motor skill. There is a physical knack to it which comes with practice. There is no substitute for detailed knowledge. That's why an expert can make it look quick and painless.

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