Schedule the evaluation at the same time you schedule everything else. It’s just as important as everything else, so why leave it to the end-of-program crunch of everybody’s schedule. Put it on your schedule at the beginning.
Then, have a tool. It doesn’t need to be a complicated evaluation; the simple questions What Worked and What Could Be Better are all you really need. Of course, “worked” means you’re clear on what your program was trying to do; useful evaluation depends on clear goals and objectives.