"How-to" essays can be difficult because you want to avoid just writing out a list of steps. Though the progression of the essay is logical on its own, somehow, you have to convert the steps into organized paragraphs.
Pre-writing, however, needs no organization. In fact, I encourage you to simply create a list of each step in order. If repairing broken cars is not something you are familiar with, you will need to find someone who is. Unless you are going to write an essay about "diagnosing car trouble," I also imagine you will need to decide what exactly is going to be wrong with the car in your essay.
Once you've got a list of steps, see if you can combine any to form some logical groups. I have no idea how many groups you will come up with nor how they will be organized, but aim to have between 3 and 5. Of course, organize them chronologically, but try to come up with some sort of a label that distinguishes each group. Then, your thesis will simply reflect the groups of steps. An example might sound something like this:
No matter what is the problem, repairing broken cars can be summed up into a simple three step process: diagnosing the problem, obtaining the necessary parts, and putting in the labor until the problem is fixed.