The game consists of whatever the players define it as by creating and playing cards. There are no initial rules, and while there may be conventions or House Rules, it is in the spirit of the game to spite and explode these conventions, as well as to adhere to them religiously.
Decide how many cards you will use. Maybe 60 for 3-4 players and 90 for 5-6 players. Or maybe less, 40 can also be good.
Use about 1/3 blanks, 1/3 stockpile and 1/3 new cards. If you don't have a stockpile, use more blanks or new cards. Remember that blanks slow down the game, it's better to spend a bit more time in Prologue than to have more than half the cards blank before commencing play.
New cards must be filled out before the game. Divide them among the players. Another possibility (for the impatient) is to place (part of) the blanks in the middle of the table, so that inspiration-deficient people can draw less new cards.
A playable card has: A title and a picture are mandatory. Optionally: a description of what the card does and/or flavourtext. The description can be as short as +300 pts, but also really complicated. Without description, the card will just sit there, wherever it is played.
The best cards are made with replayability in mind.
Shuffle. Everybody gets five cards. The rest of the cards go in the middle of the table as the draw pile. Designate a spot for the discard pile, next to the draw pile.
Player to the left of the dealer begins, gameplay generally goes clockwise.
Draw one, play one. If there are no cards left to draw, just play one. If you cannot play a card, meaning you have no blanks and no cards in your hand can be played in the current game situation, draw a second card and pass your turn. If you can't do this because there are no cards left, the game is over.
If a card doesn't mention a specific target, it can be played either on yourself, another player or to the centre of the table, in which case it affects all players. If the card has a lasting effect, it stays in front of the target it was played on (or in the centre of the table), otherwise put it on the discard pile.
A card with no description (or just flavour text) is considered to have a lasting effect and just sits there, doing nothing, except possibly when another card interacts with it.
If the card has a point value or similar, this is also considered a lasting effect, and can be destroyed during the game, losing the points or similar.
If you have a blank card, you can use a pen or something to make it playable (see above) at any time (so if you have only blank cards in your hand, you can do this before you play it). It's best to do this during other players' turns, so as to hold up the game as little as possible.
Player with the most points wins. Unless a card in play says otherwise (see Golden Rule 1 below).
After the game, you have a lot of cards that you can add to your stockpile for next time. Put all cards in front of you on the table, face up. Decide how many cards you want to keep, and let each player select their X favourite cards. Store them in a secure location.
Many players believe that having their own cards favoured during the epilogue is the true 'victory' of 1000 Blank White Cards.
Rules and instructions on cards always override rules and instructions here. Rule changes can be effects of cards, and when the card is no longer in effect neither is the rule change. Regardless, rule changes are undone after the game ends, and each game starts on a clean slate.
Any dispute on the rules is resolved by consensus. Players should weigh the intent of the rule's creator, the literal interpretation of the rule, and above all, which of these would be more amusing for everyone.
Balance the game by introducing house rules that may or may not be subject to Golden Rule 1. Some suggestions follow, pick and choose.
Limit point score effects to +/-1000 points per player. If points-cards are destroyable, this might not be necessary.
Don't get into "bigger gun" fights. A bigger gun fight typically goes: this card cannot be discarded, discard any card, even one that cannot be discarded, this card cannot be discarded, even by cards that can discard cards that cannot be discarded and so on. Instead, find a way around every problem: this card cannot be discarded, exile any card to the nearest refrigerator. players get no points from cold cards.
The goal of the game is not so much to win the game, as it is to metagame well. It is more challenging (and more impressive) to lose well than it is to win poorly. The point of the game is creativity, cleverness, and gamesmanship.
For good gamesmanship, if you plan on making a card that is "overpowered", it's better to make it during the prologue than during the game, so that everyone has an equal chance of drawing it.
Do not make cards that refer to specific players' names. These cards aren't much use when they are not around.
Cards may not prevent creation of new cards.
Alternatively: Limitations on the players' actions are themselves to be limited. Not being able to draw, play, or create cards is alright for a turn or two, but becomes aggravating beyond that. This also only applies to one card at a time.
Blanket I WIN or YOU LOSE cards aren't much fun.
If a card's action pushes any player's personal boundaries, they may refuse to participate in it.
A player cannot gain more than X turns at a time, cause another player to lose X turns at once, and can only draw X cards maximum from the deck per turn. This is usually a rule applied to one certain card at a time; combinations of cards that would produce a similar effect are usually exempt.
In practice, the card descriptions can generate rather monotonous decks of one panel cartoons bearing point values, rules or both. As conceived, the game is far broader, as it is not inherently limited in length or scope, is radically self-modifying, and can contain references to, or actual instances of, other games or activities. The game can also encode algorithms (trivially functioning as a Turing machine), store real-world data, and hold or refer to non-card objects.
This text based on various 1KBWC pages all over the Internet. In particular, a number of house rules were inspired by the 1KBWC Wiki on Wikia.com, very much recommended to check out for further inspiration.