RN
The useful answer to this question is one almost nobody publishes, because it does not sustain a thousand-word article: for the overwhelming majority of men, the honest answer is that you cannot, you will not be able to, and no amount of work will change it.
That is not a discouraging framing. It is the framing that keeps people out of a physiotherapist's waiting room.
What the numbers are, and what they are worth
Every statistic you will see on this subject descends from Kinsey's 1948 work. The findings there were that fewer than one per cent of men reported being able to achieve oral contact at all, and something in the range of two to three men per thousand reported being able to complete the act. The frequently quoted figure of one in four hundred is a later restatement of that data rather than anything Kinsey wrote.
Treat those numbers with appropriate suspicion. They are self-reported, they come from a volunteer sample that was never representative, and Kinsey's sampling has been under methodological criticism for the better part of eighty years. They are also the only figures anyone has, which is why they have survived. The reasonable summary is that this is rare — well under one man in a hundred — and that the precise fraction is not known.
Why effort is not the variable
Here is the part the encouraging guides skip.
What determines whether this is possible is mostly fixed anatomy. The length of your torso relative to your legs. Where your ribcage sits. The geometry of your vertebrae and the available range at each segment. Penis length. These are proportions, not capacities, and proportions do not respond to training.
Spinal flexibility does improve with consistent work, within limits — but the limits are the point. Range of motion in the spine is bounded by bone and ligament, and the bounded part is the part that matters here. Someone whose proportions put them two inches short is not two inches of hamstring stretching away from success. They are permanently short.
So the standard advice — start doing yoga, build up gradually, be patient, you will get there — quietly reframes a fixed anatomical question as a question of dedication. That reframing is where the harm comes in, because it tells the person who is not close to keep pushing.
The actual risk
The mechanism of concern is forced spinal flexion, in the neck and lower back.
Forced flexion of the cervical or lumbar spine is a documented route to disc herniation and nerve compression. The risk is not evenly distributed: it rises with age, with existing degenerative change, and sharply where there is pre-existing cord compression a person may not know they have. What makes this specific attempt riskier than ordinary stretching is the combination — end-range flexion, sustained rather than momentary, loaded by bodyweight, with the whole aim being to push a little further than last time.
The reassuring half of the picture is that most cervical disc herniations resolve with conservative treatment inside roughly eight to twelve weeks. The unreassuring half is that some do not, and that is a serious outcome to accept in exchange for a party trick.
The rules are simple. Sharp pain stops the attempt permanently, not until tomorrow. Anything that radiates down an arm or a leg, any numbness, tingling or weakness, is a doctor's appointment rather than something to sleep off. Never load a flexed spine — no pushing your head down with your hands, no wedging yourself against furniture, nothing that makes the position hold itself. If you cannot come out of a position under your own control, you should not have got into it.
And if you have an existing back or neck problem, or any spinal diagnosis at all, this is one to leave alone entirely.
What people are usually actually after
Worth asking, because there is often a substitutable goal underneath.
If the appeal is the sensation, that is available: it is oral sex, and a partner or a toy delivers it without spinal flexion. If the appeal is autonomy — not needing anyone else — that is what solo sex already is, and the specific act is a very inefficient route to it. If the appeal is the achievement, that is the honest one, and it is also the one that ends in injuries, because achievement framing is what makes people push past the signal to stop.
The genuinely good outcome from reading this is deciding you are not close, and stopping there. A regular flexibility practice is worth having on its own merits, for reasons that have nothing to do with this. Pursuing it as a means to this end is how a good habit becomes a bad one.
If you came here from curiosity rather than intent, the other guides cover a wide range of solo and partnered ground with better returns on the effort.