These were taken in large part from: http://www.game-club.com/cover1/mankrule.htm, however this page no longer exists. My annotations are in italics, however if there is a difference between these rules and the reference, use these rules.
A sample game is on-line.
The game itself has many variants, and can be played with nearly any amount of stones and/or pits. The most common form is played on a rectangular ceramic or wooden board with two shallow "stores", one on each end, and a dozen smaller "pits" that are positioned in two rows of six each. Either three, four, or five (depending on the variant), stones are placed in each pit, leaving the two stores empty.
We will use 6 cups per side and 4 or 5 stones in each cup initially.
Each player must choose a store and one of the rows of six pits, and take turns selecting pits.
Each player uses the 'store' on their right hand side.
Once a pit has been selected, all of the stones contained in that pit are distributed counter-clockwise to the other pits on the board. In other words, the moving player places one stone in every pit counter-clockwise to the one being emptied, skipping the opponent's store (but not skipping his/her own store!), until all of the stones in that pit have been distributed. If the last piece being distributed ends up in the moving player's store, the player may make an additional move, continuing to do so if each of his/her last stone/s ends up in the store again. It is possible to "capture" the opponent's stones by having the last stone of a move land in an empty pit on the moving player's side that is opposite to a pit with stones on the opponent's side. If this happens, both the stone that had landed in the previously empty pit, and all of the stones in the opponent's pit directly opposite go to the moving player's store.
Clarification: If you move into an empty pit on your side, and the opponenent has no stones in the opposing pit, you still 'capture' your own stone (and all of his (zero) stones in the opposing pit).
The game ends when one player's side has been emptied of stones.
If all of the pits on one player's side of the board have been emptied, all of the stones on the other player's side of the board are then placed into the store of the player controlling the "non-emptied" side. The stones in each store are then counted, and the player with the most stones in his/her store wins the game.