Process: 5 day Core course, teaching and learning, make sure you do the one with the sim and you get CRMI ticket as well. Then 5/6 day SFI course, TRI rating only issued if you have done 30 sectors on the type in previous 12 months. Check the restrictions, without mentorship from a TRTO you may have problems, but usually the TRI course providers can help. Good investment, probably about 10K in total.
Read STD DOC 24 and LASORS for requirements.
If you have a current FI rating and did it at an approved school, you may get a core course exemption, but then you may not get the CRMI privileges.
Of course you need a type rating on the type you want to instruct, but as an SFI no currency.
You don't have to be a Captain to be a TRI, just meet the requirements.
Good luck.