Simple answer:
The Confederacy needs to hold Tennessee as well as it held Virginia - at least Middle and Eastern Tennessee. If it does so, it's going to be hard to present something (obvious) enough to justify the costs of blood and treasure in 1864 compared to OTL.
Doing better in the Eastern theater in 1863 would be good as well - if the Army of the Potomac suffers as heavily proportionally as the ANV did OTL, or more so, that's going to hurt, especially if we get rid of important commanders.
Getting the Union to fire the first shot - on what? Firing on the batteries ringing Sumter? What's the point?
Getting rid of Grant and Thomas and Smith* (OTL a loss, but he needs to be one TTL) might be a big enough void in the Union high command to go somewhere.
But it's not going to be easy. The Union doesn't have any disadvantages that the Confederacy can exploit, foreign aid more substantial than the CSA being able to purchase arms is unlikely, and the CSA is fighting with a few very nasty disadvantages of its own.
*
http://en.wikipedia.org/wiki/Charles_F._Smith