Is 3rd time the charm or am I good at two, I'm pretty new to DIY construction and don't know how you guys who do it successfully actually do it. Am I being careful enough?
When you feel confident with your measurments, thats the time to machine the parts(cut, rip, route, whatever) Planning is key. Make a CUTTING BILL(list), showing thickness, width and length, and double check your parts against the plan. Your plan should show a front view, a top view, a side view, and any sections/details that are required to understand EXACTLY how you are assembling the unit. DIY can be really fun, but can also be an excercise in frustration. Take the frustration out of the equasion by looking at every detail, and see it in your mind thoroughly. Once you know EXACTLY what and how you are doing it, STICK TO YOUR PLAN.
And lastly, when first learning to use power tools, it isn't apparent what NOT to do. Like for instance. When using a CHOP SAW, skill saw or a radial arm saw, make sure the material is squarely against the fence. Other wise the blade will grab and yank the material backward. When this happens, it can scare you and possibly cause you to jerk your hands into a blade. Believe me, I've SEEN it happen. REMEMBER..MURPHY IS ALWAYS AROUND just waiting for you to be an idiot. If you aren't familiar with an operation, get the info from someone who knows, or look on the net. There are tons of forums and info sites about tools.
As far as the piece goes, DON"T compromise. Thats another habit to get into. For your own satisfaction sake, DO IT RIGHT.

Even if it cost you a bit more in time and money. You will be glad you did. Everytime you let something go that is funky, you slowly lower your craftsmanship. Like the saying goes,
....good enough for government work.....well, the hell with the government.
BTW, depending on the project, sometimes I'll buy enough material to cover a mistake on the LONGEST piece I'm cutting. That way I save a trip, and, over time, you end up with a supply of various types of materials. Down the road, they come in handy. But like I said, it depends. I don't make many mistakes with this stuff anymore, as I've done it for so long, AND I'm a CAD draftsman.
The thing you DO NOT want to make a mistake on, is UNDERESTIMATING what a tool can do. Both in the positive sense and the negative sense...as in CUTTING YOUR HAND OFF!!

BE CAFEFULL. I've seen so many people hurt in shops it would make you sick. Even simple tools such as an electric drill can do SERIOUS damage. I know from experience. Almost removed all the meat from one finger when I tried to enlarge a hole in a piece of sheet metal while holding the piece with a bare hand.....how do you spell M O R O N!! It grabbed the metal and pulled my finger up agaist the 1/2" bit while wrapping the metal around it.........HOLY SHIT did that hurt.
fitZ
PS. One last thing. ASSUME NOTHING! Especially with power tools.