Let's say a Founder currently owns 100% of the company but wishes to offer 10% of the company to a part-time contributor. He offers a 4-year vesting period to that contributor with a 1-year cliff.
So far, easy enough.
But, how is this constructed practically? He can't offer his own shares as this would be deemed to be personally selling them. Should an option pool be created so that he now holds 90% of the company with the 10% being in the option pool and a quarter of this option pool given to the part-timer every year? But wouldn't that mean that any unvested shares return to the company (which by then may have more investors) and not the founder?
You need to create an option pool by creating a stock option program and issue more shares, thus diluting your original ownership. This would be kind of like a "round" of funding. Of course, this all assumes you are a C-corporation and can do that.
The un-vested shares would not be issued and the total ownership (percentage) would be adjusted accordingly.
The pool would still be considered in the fully-diluted number of shares outstanding but if not issued, it would not matter that much.
First, make sure your other co-owner is ok with offering incentive stock options to a third person.
Then talk to a lawyer that works with startups. He'll knows how to take care of these details and what the options are.
You can alway take a look at StartupLawyer. His articles are very short an concise, maybe they'll help you figure some details out by yourself.