Smart Bombs are actually the easiest thing to SDI in the game...
These were the drills I was doing when I was messing with this a few weeks ago:
1. To practice SDI in one direction, use a Smart Bomb. With good SDI, you can fall out of the explosion and escape it before the final hit.
2. To practice SDI in multiple directions, use a Smart Bomb near a ledge and
go around and under the stage to tech the stagespike.
3. To bring it all together in a fun and flashy way, you can give a CPU Mac a KO Punch and try to SDI
down and under the ledge to tech it and save yourself; CPU Mac is pretty predictable about when he punches, and the slowdown gives you more time to mash out your SDI (though it's pretty strict, and the camera zoom makes it hard/impossible to see what you're doing)
If you just wanted to practice it in more practical situations and have another controller/someone else available, I'd practice escaping things like Little Mac's rapid jab.
Lastly, it might have to do with the direction you're SDIing in; I couldn't really find resources that accurately explained what to do, but wiggling back and forth over cardinal directions resulted in a lot of movement while diagonals resulted in little to none. Someone more familiar with the mechanics than myself could likely better fill you in on that.