This is not legal advice, and I am not a lawyer.
"Defining the process for levelling up" I have always found to be a bit vague. It doesn't include providing an XP chart-- you can do that. It doesn't include providing a class description, or a class chart, showing when certain benefits are gained-- you can do that, too, and many have.
And I think as long as you let "experienced" players figure these things out for themselves, you'll be ok.
When you start describing how to compare a PCs experience points against the table, how to roll new hit points, how to assign skill points, how to choose feats, and yes, perhaps, even how to apply Improvement Points, you might very well be crossing into the area of "defining the process." I think any kind of step-by-step delineation of how to advance from one level to the next would run afoul of the restriction.
Wulf
