Playing the "ghost" is a great way to keep solo practice interesting and competitive, and you can use it for 7-ball, 8-ball, 9-ball or 10-ball - and even the game of rotation, where you have a full rack and try to sink the balls in numerical order.
Here are the basic rules. You break the rack and then get "ball in hand" for your first shot, meaning that you can place the cue ball anywhere on the table. From there, your goal is to run the whole rack (or, as in 8-ball, the solids or stripes, plus the 8). You must win the game during this turn at the pool table. If you miss or scratch, it's considered a win for the "ghost," your imaginary opponent who automatically runs the table whenever he gets the chance. As far as the endpoint, you can play to a certain number of games (say, 10, for example, so the final score might be 8-2 or 6-4, etc.), or you can play until you or the ghost reaches a certain number of games (called a "race," as in "race to 7").
Professional players win games against the ghost in excess of 80 percent of the time, and a beginner will struggle to win 10 percent. If you are a beginner, your immediate goal with ball in hand should be to sink three balls every opportunity. You'll quickly get a feel for planning your cue ball position for the next shot.
Those are the basics, but you can add any number of wrinkles to the rules and scoring. For example, the basic rules do not take into consideration safety play, where you end up with a difficult or impossible shot on your next ball and you instead decide to execute a defensive shot. That's when you contact a legal object ball, and the cue ball rolls to a position where your opponent has no clear shot for his next turn. If you want to include safety play, there are several options. If your safety is successful, and the ghost has no clear shot for his turn, you can call it a draw and move onto the next game, or you can give yourself ball in hand again and continue your runout. (Typically, though, you would limit yourself to one safety per rack.) Or you could execute your safety, and then actually take the next shot for the ghost (and taking an honest whack at it). If you, as the ghost, sink the shot, you've lost the rack; if you make contact with the object ball but miss the shot, you change back to your original self and keep going, etc.
It's also up to you what to do if you scratch on the break shot. You can consider that a game loss, or you can spot the balls that went down on the break (i.e., put them on the marked spot where you usually place the 1 ball for a new rack) and start as you normally would with ball in hand.
Some players use a variation of playing the ghost in which they count the number of balls they sink in each inning, and then divide that total by the number of games played. So, if you're playing 10 games of 9-ball, and you sink balls 1 through 5 in every rack, you'll have sunk 50 balls total, for an average of 5.0 balls. You can use that number as a rating to keep track of your progress. You can use the same concept in 8-ball, where you'll be shooting for eight total balls (the solids or stripes, plus the 8), except you're not sinking the balls in numerical order.
(Thanks to BD columnist Mark Wilson, and the posters on the "Billiards Digest Cue Chalk Board" Forum.)