So, you want to become a measurer!
You should go to http://www.usatf.org/events/courses/measurers/ and find how many measurers there are in your area. If there are too many in a small area, then there may not be enough work to justify purchasing the counter and supplies. But, it depends on how you want to market yourself. Some folks measure many courses, some measure few. Look at the maps the measurers in your area produce (you find those in the listing for measurers), to see if you have the skill/software to produce maps at least as good, if not better, than what is being produced. Race Directors like good maps, and gravitate towards good measurers who make good maps.
For clarification, you are endeavoring to become a RRTC Measurer, which is different than a Certifier. Anyone can measure, as long as they follow the procedures in the manual: http://www.usatf.org/events/courses/certification/manual/. A Certifier is someone who reviews other measurers' work, and issues the certificate itself.
The first step is to download the manual, and read it thoroughly. It explains the details of what we do, and the detail involved in a measurement. It is not difficult, but it does require attention to detail. Someone who just goes through the motions is not a good candidate for becoming a measurer.
Once you have read the manual, if you decide this is for you, then you need to purchase a Jones Counter (www.JonesCounter.com). I like the 5-digit, as there is one less digit to remember as you are riding to your next split location. The rollover of what would be the 6th digit is not a problem, as it is easy to determine what the 6th digit would be. More on that later, if you need further explanation.
Put the counter on your bike, and pick a course to ride in your neighborhood. It doesn't matter the length, but you do want multiple left- and right-hand turns. Note the counter when you begin, then note the count when you finish. You can also stop at certain spots during your ride (don't do too many, just one or two). Compare the number of counts required for each ride (ride both directions), to see if you are riding a repeatable course. If your clicks are more than 5 counts off on a course less than a couple of miles, you need to work on sighting and riding the Shortest Possible Route (SPR). SPR is critical in course measurement. You must ride a repeatable SPR, so your click counts are within tolerance.
Once you have gotten proficient in riding a repeatable SPR, you can find courses to measure. I started contacting races that didn't advertise that their course was certified, and suggested they have their course certified. The reasons for certifying are explained on this page, and also here, about the middle of the page. Race directors constantly are commenting on how many more runners they get, once they start advertising that their course is certified.
Hope this gives you enough info to look into it more.
Let us know if you have questions.