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
Prior to NetHack 3.6.0, players had to set their own rules and use several workarounds such as wearing blindfolds or permanently polymorphing into blind monsters. Few roles made the necessary items available from the start of the game, especially without start scumming, and even then you would be momentarily non-blind for a short time at the beginning to blind yourself.
NetHack 3.6.0 adds the blind option to be set in the configuration file. This will make the player permanently, incurably blind (using the #conduct extended command will display "You have been blind from birth"), which is perfect for a zen game since it prevents the player from accidentally breaking conduct. Additionally, it is now possible to read the Book of the Dead while blind, because you cannot win the game without doing so.
The difficulty in zen is that it is like a mixture of other conducts with additional twists to make it even harder. While blind, you may not read, though you may still use scrolls after you get the shopkeepers to name them to you (you only need to know those magic words). You cannot use altars to find the BUC status of an object because you do not see if the objects glow and in what color. You also do not see what your objects look like by their material or color, so all potions are only shown as "a potion", wands as "a wand" and so on.
A zen player will find exploration slow and tedious, especially in the early game. Players are recommended to use the search command with every step, or almost every step. 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. Moving blindly will anger a peaceful creature if you attempt to walk onto them without "seeing" them, even when using the safe-move function m. This is extremely important when dealing with shopkeepers while still lacking telepathy. A zen tourist will have a slightly easier time passing through the gnomish mines by using the starting scrolls of magic mapping, and it is recommended that they save the scrolls for that purpose as standard dungeon levels are easier to explore. If you know (or reasonably suspect) there is a wall, then walk into it. This will reveal the wall on the map without using up a turn.
The most challenging part of a zen game is the very beginning. The player must fumble about the dungeon with no idea of the location of monsters, and no identification of those monsters they do locate. Often, your first indication of the presence of a monster will be "It hits!" A player's primary goal will be to acquire a source of telepathy as soon as possible, and the most likely source is a floating eye. Thankfully, zen players are blind and will thus be immune to the floating eye's paralysis. A floating eye has a difficulty rating of 3, so will begin appearing when the average of the player's dungeon level and experience level is 3. A player should use that information to their advantage by not proceeding much farther into the dungeon beyond what is required to generate a floating eye. A wizard fortunate enough to start with the detect monsters spell will have a much easier time locating an eye, but for the rest this will be blind luck. No pun intended.
Avoid eating dwarf and gnome corpses until you acquire telepathy. It could be a zombie corpse! (You "destroy" zombies, but "kill" living dwarves.)
Alternative sources of telepathy are prayer with at least 6 Luck, a helm of telepathy, or an amulet of ESP. Getting enough Luck without a luckstone can be challenging in the early game, and the two items that grant extrinsic telepathy are not particularly common, so not many games will have this as the first source of telepathy.
It is advisable for a zen player to start without a pet, set via the options file. A pet is extremely difficult to keep track of and distinguish from hostile monsters in the early game, and a player will run a serious risk of killing the pet accidentally. An alternative is to quickly ditch the starting pet on DL1 and proceeding into the dungeon, returning to retame the pet when telepathy is acquired. Eventually, a zen player should acquire a pet if they wish to steal items from a shop.
A lawful or neutral zen player should be especially careful of committing accidental or intentional murder, as this will cause the loss of intrinsic telepathy.
For information on how you can see objects while blind, see Blindness#Object appearances, which contains a full list of ways to make an object "seen" without breaking the conduct.
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.
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. Since bumping into a shopkeeper will anger them, even when using the m command, which nearly always results in your death, a player is best advised to simply avoid shops until they can properly use them.
Selling items works as normal.
Shops will be 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 Eyes of the Overworld
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.
NetHack 3.4.3 specific strategy
Only samurai, rogues, wizards, and tourists can start with a blindfold or towel, and therefore attempt zen conduct. Start scumming for one is not cheating. Generally, a samurai is considered the easiest role for 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. A rogue or tourist is going to have a 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.
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 allows use of the Book of the Dead. Unlike the option introduced in 3.6.0, the conduct cannot be broken by the Eyes of the Overworld, which simply no longer grant astral vision.
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 of these roles. A nudist zen ranger has been ascended in UnNetHack, which allows all roles to try zen.