How can an experienced project manager start a career in teaching PMP courses?

Experience is one of the most valuable assets you can have.
Theory is something you always can learn from books and courses however does not always apply to practise.
As a PM you may take two different directions where just one is PMP teacher.
Firstly, you may offer your services as exam prep courses.
Start from preparing course agenda and training material so it matches the PMI requirements.
Create website and take care about SEO to position it well.
Start offering free classes to dry-run your training plan using WebEx or simillar virtual meeting software available in the Internet.
Once you verify your agenda and plan works and you receive positive feedback from the audience you may start planning another sessions for a fee.
Secondly, using your biggest asses which is experience, you may want to think of some kind of consulting serivces or advisory board, charging membership fee.
Each member would need to pay a fee to post questions regarding this matter and you'll be answering them.
This would require more thoughts to have it accomplished but I think it can be a good business idea.
I hope this helps.