I've actually been cutting my own hair for probably close to 15 years now with a standard hair clipper. It's not just a buzz-cut either. I actually have more on the top (like a #7) and a less on the sides (#3).
It really just comes from practice and learning the shape of your head. Once you use clippers for a while you'll understand how it works. The only challenging part is trimming your neckline which is where I usually have my wife help me. Otherwise it's just a lot of practice.
One tip is always start with a higher numbered attachment and work your way down. It's better to cut off less than more when you're first learning. If you don't like the result, you can go to a barber and he can fix it for you.
Red, I use #3. Green, I use #5. Blue, I do #8 at the front, #7 towards the back. Then, after that, I use the inbetween numbers. So I use a #4, and smooth out the transition from red to green. Then I use a #6, and smooth out from green to blue.
To finish, thinning sheers. They're basically magic. I have thick hair, so I just clip all over the place at different lengths, and it blends everything together nicely.
The neckline is difficult to get straight. I'm finally able to do it with a few mirrors, but it's a lot easier if you have someone else that can take 10 seconds for you.
I just use some clippers by Wahl, and they've always worked fine. Cleanup takes the most time. Watch a few YouTube videos to get a better idea, and pick a forgiving time to cut your hair as a first attempt. If all goes wrong, you can just buzz it, or wear a hat for a week.
It really just comes from practice and learning the shape of your head. Once you use clippers for a while you'll understand how it works. The only challenging part is trimming your neckline which is where I usually have my wife help me. Otherwise it's just a lot of practice.
One tip is always start with a higher numbered attachment and work your way down. It's better to cut off less than more when you're first learning. If you don't like the result, you can go to a barber and he can fix it for you.