How do I read an index wager line?
Let's look at an College Basketball index wager, although index wagers are available for multiple sports.

San Francisco vs. Gonzaga	Sell	Buy
Gonzaga Winning Margin	12	15
Total Points	139	142

In the above example, there are two potential wagers, each seperate from each other.
For the Gonzaga winning margin, the "Buy" means you think the margin of victory will be higher than 15. The more Gonzaga wins by, the more you win! If you sell, you think Gonzaga win by 15 or less (or San Francisco will win).

For the total points, the same principle applies. Buying at 142 means you think more than 142 points will be scored in total. Selling indicates you feel less than 139 points will be scored.

In each of these, it is not only the wager that matters, but the magnitude of how right you are!

How do I bet and score an index wager?
First, to make a bet on an index wager, decide whether you are going to buy or sell at the given values. Then decide how much you wish to risk per point. For simplicity, let's say you wager $1. Let's say you wish to BUY Gonzaga $1 at 15. Every point difference from 15 now plays a role. If Gonzaga wins by 25, you are +10 units. In this case, you would win $10 (10 units x $1). If Gonzaga only won by 5, you would lose $10 (-10 x $1). Total wagers have even more flexibility since totals scores can vary further from their buy/sell point somewhat easily.

What if I am very right... Or very wrong!
Typically, when index wagering you have a stop loss or locked amount. This prevents you from losing everything on one wager. What if Gonzaga had a bad night and lost by 30? You would be -45 units! Typically, you set a stop loss is about 25, meaning you can only win 25 times or stake... or lose 25 times your stake - in this case, $1. When placing index wagers, the sportsbook always puts your worst case loss in a holding account until the event is graded.