Unlike most other conducts, you must enable the blind option in your configuration file with the following line in order for your game to qualify. This makes you permanently blind without requiring other sources of blindness.
NetHack does not actually use the term "zen" anywhere; it refers to the conduct as blind from birth.
Even when you are permanently blind, you are able to read (or at least "turn the pages of") the Book of the Dead, since otherwise it would not be possible to win the game, similar to how reading it does not break illiterate conduct.
The difficulty in zen lies in the fact that you cannot identify the appearance and beatitude of most objects, and they are only shown as their object class - all potions are only shown as "a potion", wands as "a wand", etc. The exception to this is your starting inventory, which you always start with knowledge of. You cannot read scrolls while blind unless you learn their labels, e.g. buying scrolls from shopkeepers will have them name the labels for you. You also cannot use altars to find the BUC status of an object, because you cannot see if the objects glow and in what color.
Exploration will be slow and tedious, especially in the early game. Moving blindly will anger a peaceful creature if you attempt to walk onto their square without seeing them, even when using the safe-move function m. A lawful or neutral zen player should be especially careful of committing accidental or intentional murder.
Players are recommended to use the search command with almost every step at least. This will be the only way to get any sort of warning of an approaching monster, and it will speed up the "mapping" of the level as the search command will check each adjacent square for walls, items, or creatures, while just moving will only check the square you're about to step on. If you know (or reasonably suspect) there is a wall, then walk into it; walking into a wall tile will reveal it on the map without using up a turn.
A player's primary goal will be to acquire a source of telepathy as soon as possible - the most commonplace and likely source of telepathy is a floating eye, which will begin appearing when the average of the player's dungeon level and experience level is roughly 3. Players seeking a floating eye should not proceed much farther beyond the dungeon floors most likely to generate one - a Wizard fortunate enough to start with the detect monsters spell will have a much easier time locating an eye, but the rest will be forced to rely (appropriately) on blind luck.
Players can also obtain telepathy via prayer with at least 6 Luck, a helm of telepathy or an amulet of ESP, but getting enough Luck without a luckstone is challenging in the early game, and the other two items are not particularly common.
It is advisable for a zen player to start without a pet, also set via the options file. A pet is extremely difficult to keep track of and distinguish from hostile monsters in the early game, and the player runs a serious risk of killing them accidentally. An alternative is to quickly ditch a starting pet on DL1 and proceeding into the dungeon, returning to re-tame the pet when telepathy is acquired. In either case, a zen player should eventually acquire a pet to identify the BUC status of items.
For information on how you can see objects while blind, see the article section on object appearances, which contains a full list of ways to make an object "seen" without breaking the conduct. Most notably, a blessed potion of object detection, which can be dropped by a nymph, will identify all items in your inventory, containers and the map. If the farlook command ; does not tell you the appearance of an object even though you should know it, the object is buried or carried by a monster, or has since been moved away.
Be careful not to eat zombie corpses, and keep an eye out for critical messages in this regard when fighting monster - you "destroy" zombies, but "kill" living enemies. Zombies also cannot be seen via telepathy.
Shops are important for identifying the appearance of large numbers of items, especially scrolls and potions. A container to place the items in and a pet to steal the container back will speed up the process of re-acquiring items you've just sold to the shopkeeper for identification.
The pay command only works if you can see the shopkeeper, either by telepathy or monster detection, or if he is angry. Alternatively, you may drop enough gold to cover your purchase and then teleport, jump or phase out, even without teleport control. The practical result of this is that a player will typically have no way to get an item out of a shop until telepathy is acquired. Bumping into a shopkeeper will anger them, even when using the m command; since this will nearly always result in death, a zen player is best advised to simply avoid shops until they can properly use them.
Selling items works as normal, which might be useful if you get burdened before acquiring telepathy.
Water or lava tiles cannot be identified while levitating, so be careful on water levels such as Medusa level and use magic mapping if possible.
Do not step on a cockatrice corpse while not wearing gloves.
Do NOT, under any circumstances, attempt to wear the Eyes of the Overworld in a zen game. Putting them on will produce the message "For the first time in your life, you can see!" and break the conduct. They are the only thing in the game that can do this. Because the Eyes only provide effects when worn, they aren't even worth carrying around.
Only a handful of people are known to have ascended zen games, and most of them have been zen samurai. this post to RGRN announces the first known ascended zen tourist. João Santos is the only person known to have completed zen ascensions in all four pre-3.6.0 roles able to do zen. A nudist zen ranger has been ascended in UnNetHack, which allows all roles to try zen.
In NetHack 3.4.3 and earlier versions, players had to set their own rules and use several workarounds, such as wearing blindfolds or permanently polymorphing into blind monsters. Only Samurai, Rogues, Wizards, and Tourists can start with a blindfold or towel, which typically required a little start scumming, and even then you would be momentarily non-blind for a short time at the beginning to blind yourself.
NetHack 3.4.3 specific strategy
Generally, a Samurai is considered the easiest role for 3.4.3 zen conduct due to their good starting armor and strong melee combat skills, which are important in the early parts of the game. A Wizard can start with several helpful items for the early game as well, such as a wand of fire or lightning for partially reliable engraving of Elbereth, or a ring of warning. The Rogue and Tourist are going to have a more difficult time, due to weaker melee skills and a lack of strong starting gear.
A zen game is flawed if a nymph or monkey steals your blinding item and you immediately put on another one, without looking at your inventory. Should this happen, be careful not to use the #name or i commands, since these will reveal object appearances in your inventory.
The conduct is named for a school of Mahayana Buddhism that originated in China during the Tang dynasty. Zen Buddhism emphasizes rigorous self-restraint, meditation, and a direct understanding of "natures", specifically the "nature of mind" and the "nature of things", and encourages the expression of these insights for the benefit of others in daily life:
The Dharma is the truth that all natures are pure. By this truth, all appearances are empty... The buddha is your own mind... As long as mortals are attached to appearances, they're unaware that their minds are empty. And by mistakenly clinging to the appearance of things they lose the Way. -Bodhidharma
Zen conduct in NetHack focuses more on the "detachment from appearances" tenet of the above.
UnNetHack adds functionality that allows players to optionally select certain conducts at the start of the game, tracks them, and punishes players for breaking them. If the player selects Zen Conduct, they begin the game wearing a cursed blindfold, that, if ever removed, breaks the conduct and summons some monsters around the player. To use the Book of the Dead, you can #invoke it.
NetHack 4 offers a "permablind" option that gives blindness as an intrinsic, overriding any attempt to remove it. Like UnNetHack, #invoke also allows use of the Book of the Dead. The conduct cannot be broken by the Eyes of the Overworld, because they no longer grant astral vision.