Project Management techniques in use today are difficult to apply directly to something as small as fixing one car.
Project management is a term used to refer to a number of different techniques that can be used to make sure a project is done on time and at the proper cost. I would narrow this down to 2 main types of project: construction and science-y stuff. We'll talk more to construction, as it is a far more direct comparison than science-y stuff.
Construction projects work VERY well with modern earned value systems. RSMeans publishes cost books every year that average how long it will take X workers in Y location to accomplish Z task for $xx,xxx dollars. These are industry standard, and surprisingly accurate. They are used before starting a project to plan out the whole thing. If the construction company cannot keep to the baseline schedule that they agreed upon using these data, they usually have to make up the difference out of their own pocket. Barring extenuating circumstances.
A house is about the smallest thing that I can think of that requires Project Management. A wide range of workers, conflicting tasks, materials, specialists, and a deadline; something is needed to keep all of these in line and on time. You can't build the frame until the foundation is poured. The electricians need to come before you drywall. if you get pushed too far back, your roofers might not be availible. Things need to happen at certain times based on where we are in the project.
Compare this to working on a car - 1 unpaid worker (you), doing all of the work, parts, no real gonna-get-fired deadline. AND its not your primary source of putting food on the table (based on the "DIY" part of the question). There are some things that have to happen in a certain order, but not nearly the level of interconnectedness that a construction project has. Unless you are starting with the rights to a VIN and a dream, a car is too small a project to apply "Project Management."
So What Can I Do?
We can take a lot of lessons from Project Management and apply them on smaller scales. I am misusing some terms here for understandability, don't get mad if you are a project manager.
Deadline -- All of what I am going to say is based around creation of a deadline. At the end of the day, human beings are easily distracted creatures. You need to be honest with yourself and come up with some kind of vaguely binding contract. Have your SO hold you to it. Get some friends that are going to come check up on you. Set calender reminders. You can't have a beer till you get X done. Something. Once you are beholden to some power greater than yourself:
1.) Planning -- When you are planning out your project, you are going to want to take a few things into account. Draw this plan out. Write it down. Make it something that physically exists.
- Uncertainty. The longer your project, the more life is going to pop up and get in your way. Don't make plans that you know you are going to be unable to stick too. Guess how long something will take you to do, and double it. This way you have some extra time built in to deal with life and ordering tools you don't have.
- Risk. Take some time to consider how this project could metaphorically blow up in your face. The car could get hit by a meteor; huge setback but low probability. You could break a bolt off; slight setback, much more probable than meteor strike. Write down some of the big risks and be ready for them. This way when they pop up, they will be FAR less surprising/heart wrenching.
- Overplanning. Be honest with yourself. You aren't gonna work on it every day of the week. You aren't gonna spend 6 AM to 10 PM on a Saturday. Set reasonable chunks of time at intervals you can actually commit to.
- Underplanning. Don't set your intervals too far apart though. If you go much more than a week without doing anything, you are going to lose motivation to continue.
- Feelin good. Every time you get something done, its a relief. Build in some easy stuff in intervals, and check it off when you get it complete. It will help to deal with the long, difficult parts.
- Unknown Unknowns. Something is going to pop up that you haven't ever seen or heard of. You don't know what these things are, and they are going to push your schedule back. Accept this. Redraw your plans when this happens. It isn't a failure, you had no way of knowing. Accept, re-plan, move forward.
2.) Executing -- With your plan planned, its time to execute. This is the hard part that falls vaguely into the realm of opinion, I feel. We built in time for life and problems and even a little bit of saying "meh, dont feel like it tonight." The construction workers need to come to work tomorrow to get paid. This is powerful motivation. You... don't. Which brings me back to the deadline I mentioned earlier. Beholden to your SO or friends or beer, you are about it. You will get something done. Even if its just a little bit, you are still working on it. You go from the neighborhood junk collector to the person that gets sh*t done.