This can be done in Practice mode. You can repeat the same combo of a character three times in a practice session. Every character has a predefined 10-hit combo in their move list. Some actually do just 5-hits, but they are called a "10-hit combo" -- so they count regardless. You do not even have to hit an enemy with those moves, just perform them. One very easy combo is from Katarina. She has only one 10-hit combo in her move list, and it is very simple to perform. Go to the move list during a practice match, select it to view a video of the combo, and then mark it to view the button prompts on-screen. You can also try it with other characters if desired, as many of them have easy combos.