You just have to practice it. It's pretty tight timing.
First try this: 112xxFireballxxRun,113, which leads to a full combo.
So, for this example, you do 112 cancelling into fireball (you can input the fireball before the 112 comes out I believe), and then as soon as it's charging the fireball run cancel for a very short time and do 113. Keep trying until you can get it to connect, you will know if your combo goes up to six hits after the 113.