You will get a random assortment of pieces in a random order. Click on the empty squares in the sewer to place the next piece. If you get a piece you don't want or can't use, you can discard it -- but be careful, because you can only discard a certain number of pieces per level. Once you have placed all the pieces, you can turn on the sewage flow -- but if your sewage system isn't closed, you'll flood the sewer and have to start over!
It's okay to have extra pieces in the sewer, as long as they aren't attached to your pipe system. If your sewer system is complete, you score points based on the number of pieces in your solution. The more pieces you used in the completed part of your system, the higher your score; any extra pieces laying around the puzzle subtract from your score.
You can restart any level at any time, unless you've already finished the level. You lose 50 points for restarting the level. When the game ends (and ONLY when the game ends!), you win neopoints based on your score. The higher your score, the more neopoints you get. Neopoints are awarded when you finish the last level, or if you quit the game. You need to score 5,000 points before you will be awarded any neopoints -- the Neopian City Council doesn't reward half-efforts, so get to fixing!