... you can take 100 rejected claims and see if you can process them ...
I don't think there is enough information in your comments here to respond properly, so I'm going to propose the following. Adapt it to fit the specifics of your situation.
1. Linda listed all the things you must do to be somebody's biller. Don't bother with those things for now. Rather, sell yourself as a consultant for a specific project (working 100 rejected claims).
2. I assume someone has been doing the provider's billing - even if it is the provider himself (not likely if he is pulling in over a million per year).
3. Sell your services to the provider at $35 an hour. Promise to make recommendations to his biller - and let the biller be the one to actually re-process the rejected claims. (This configuration puts you in the position of detective and advisor, not biller)
4. Get the EOBs for the rejected claims and see what the codes are for why a given claim was rejected. If you have an Excel spreadsheet, use that. One column for patient account number, if any; one column for last name of patient; one column for primary rejection code; one column for short description of primary rejection code. This way you can sort on any of the columns and develop an idea of the kinds of groupings you have. That helps with giving the provider feedback about what might need to be changed in his coding process or billing process. e.g.: Do you have 100 different rejection codes? Do you have 75 rejection codes of one kind, with the remaining 25 rejections divided among 4 other rejection codes?
5. Bundle like rejection codes with like. Place a note on each bundle that briefly describes what is needed before the claims can be re-submitted. That information would come from the rejection code.
6. I assume you can do Points 4 and 5 with relative ease. If you cannot, then you have a clue about what you need to work on. If you cannot, you might tell the provider that you will work for free if he lets you assist his biller in the doing of real-world stuff (process knowledge) versus just learning things from textbooks (content knowledge).
7. If you run into rejection codes that you cannot figure out, come back here and state the codes you need help with. Someone will no doubt show up to help you come up with a proper approach for resolving the issues surrounding a given rejection code. (Of course, there really is no proper approach if the reject code is "lack of timely filing".)
Sell yourself in this way to other doctors. If they take you on (for a specific project), you will increase your understanding of the billing process. And, if you are any good, you will develop some word-of-mouth buzz from the providers re. your abilities - which will be useful in gaining clients of your own as an independent biller.