The main thing is to use it high up in the air...the closer to the sky, the better....
you get the best results when all of your opponents are 80+ damaged...use your smash high up and You'll kill all that get sucked in...while they are re-spawning, heal yourself with the leftover items, and you should be straight...
note that the more your opponents are damaged the more range you have to suck them up...for example, a mario that has 18% damage has to be fairly close to you to get sucked up...in contrast, a mario with 90% damage is much easier to snag from a distance...
in any case, try to get as close to your opponents as possible...kirby's smash requires patience and precise timing to pull off...
You'll only get better with experience...just practice in training mode and memorize how much range you have vs the damage they have...