Let your child brainstorm. Initially, he might have his heart set on something entirely impractical, like a robot that does all his chores and homework for him, with a built-in television. That's fine, it doesn't need to provoke an argument yet. It is important to let him get these ideas out of his system. Help him make a list of everything he can think of, and allow him to get excited about his new assignment.
Narrow down the choices. Try to lead your child in a practical direction. Pick something she might use in her everyday life. Does she have a daily chore, such as washing the dishes or picking up the toys in her room? Try an invention that helps her scrub silverware faster, or makes cleaning up more fun. Does she ride her bike to school every day? Try to think of a way to turn her ordinary bike into a new creation. Keep her interests in mind. Can she make improvements on her dollhouse or maybe invent a new board game?
Make a plan and gather the supplies. How exactly is this invention going to work, and what exactly do you need to buy? Here is where you need to maximize your involvement while minimizing your input. Ask your child lots of questions and make note of his opinions and ideas. You want this to be his project, not yours.
Break up the assignment into small bite-sized pieces. Depending on what your child is inventing, this assignment may seem daunting to him--or even you. Dividing up the work into small portions will turn the mountain of work into a mole hill. Make a calender that shows him what he needs to accomplish each day, and make sure he sticks to it.
Supervise your child as she assembles her new invention. Step in if anything looks too dangerous or tricky, but allow your child to do as much as she can on her own. Allow her to learn from her mistakes. Don't immediately correct her if she is doing something wrong, this can often cause unnecessary fights. Let her find the problem herself, and offer help if needed. The more work she invests in her invention, the more pride she will have in it.
Test out your kid's finished project. Is it working the way it should? Make sure everything is perfect before it gets handed in for a grade. Also, make sure every part of the project is complete. Did the teacher require any written homework to explain what the invention is or how it works? Sometimes children will stuff these papers in their backpacks and forget about them, so be sure to check.