The process for achieving IT status in a second discipline is much easier than the first, all you need to do is lead an ICW in the new discipline as the co-teach requirement is waived.

1. Gain extensive experience as an instructor over two years, ideally 6 or more skills classes with 3 at your highest level.
2. Build your network, teach with multiple instructors to learn various techniques.  These instructors will also be helpful for your required 3 reference letters.
3. Line up a trainer to work with and complete the IT Registration Form
4. Complete the IT Self Study. It doesn't hurt and ACA/SEIC policies change periodically.  The office will let you know whether the online exam is necessary.
5. The ITE will be contacting the SEI Department to ensure they don't have any issues with your candidacy.  Make certain you have stayed current on your skills/assessment class reports - they are vital.
6. I strongly recommend assuming as much control in your lead as your ITE permits.  You want a very active hand in recruitment, interviews, course outline development, student evaluations, etc.  Complete the lead ICW course paperwork and make certain you gather student evaluations from all Instructor Candidates - these are essential.  Here are all of the ICW forms: IT Forms.
7. After submitting the ICW paperwork, complete the IT Application. 
8. Make certain this paperwork is complete: 3 letters of recommendation, complete ICW course report with all student evaluations and instructor trainer evaluations, a course outline that covers everything in the instructor criteria, dues are current as well as First Aid and CPR certifications.
9. Contact the SEI Department and verify all papers are in order.
10. The Standards Committee will evaluate and the SEI Department get back to you with a decision.
11. Assuming you passed, check to see that your new status is reflected on the ACA website.