As eligibility to work as a trainer, you should have at least 3–4 years of experience in the core domain sector and a good level of practical knowledge, along with the FI certification (which you have already done).
For this, it is advisable that you should first do Job and work as a SAP FI consultant for some years. This way you would gain knowledge in all areas of your module.
Take care of these points:
Do Job for at least 2–3 years as FI consultant
Work on multiple projects
Do full end - to - end configurations in your projects
Work on many real-time issues and scenarios
After working as an FI consultant, grow further and work as SAP S4 HANA Finance consultant.
When you feel confident you may start training people.
Before going to teach your subject, you should become an expert yourself. To answer the questions of your trainees and resolve their queries, you to be fully prepared.
The next step to SAP FI is SAP S4 HANA Finance which is the most popular module presently.
Since SAP FI is the first step towards SAP S4 HANA Finance, you should first gain its job experience first as I mentioned above.
Then you should go for a good SAP S4 HANA Finance Training.
After this, work as a SAP S4 HANA Finance consultant for some years. You can start training on SAP FI while learning SAP Simple Finance also.