My process basically is threefold.
Uno: Receive brief and either crap my pants (hypothetically) or receive brief and be stoked on the idea of the project ahead.
Deuce: About midway through the allotted time for completion of the task at hand. I will begin to doubt basically everything. Either the stuff I have done I believe to be trash. Or the stuff I have not yet done is sure to be trash. I need something amazing/different or ill fail pretty chronic like.
Three: Later on there always comes a point where I go. "screw it, I am just going to do it and see what happens". Because i usually over analyze things and try to do things that i probably shouldn't. Or is not my style. Or try to be like other artists i have seen.
In the end after much doubt. Step three will usually set me straight and in most cases i will turn out with something that i dig.