You go to the second command window (press left on the D-Pad), the one with Limits, Summons, and party Change. Choose a Limit, and keep mashing 'X' with the controller, until the command with Triangle is no longer "Break," which breaks the combo. After that, you should be able to do unleash a long series of attacks whilst Sora is unable to be hit.

Trinity Limit with Sora alone is EXTREMELY different from Trinity Limit with all three members.