I don't love taking mine. I think it's a fact most women don't, from the mere results we get. We get all judgey on ourselves and start fretting. Then, it gets more confusing if you don't fall in the standard sizing chart on the back of patterns. (Don't worry, there are solutions to that!) But I feel it's high time we stop being mean to ourselves and just be honest.
Now, soap box away, measurements are gonna help you pick your size. Simplicity has an excellent tutorial on all the measurements and how to take them. Plus, there is a downloadable brochure with a measurement worksheet, how to choose your size, and general FAQs about sizing. Get it here.
Keep in mind a couple things, 1) you need someone to help you take the measurements, and 2) it helps to wear the same undergarments you will wear with your project
Then some of us have other issues, uh, endowment issues, that might cause confusion in the sizing stage...but there are ways to make it better!
If you have a Full Bust, have you ever noticed on shirts gaping, or extra fabric by the armpit in front? You probably need a larger dart to take away some of that extra fabric. The method to fix it is called a Full Bust Adjustment. You can find tutorials all over the web on how to fix this problem. One of the best ones I found, that would work great with this pattern is here. A nice, simple video tutorial.
I personally don't have the Full Bust problem, more like the opposite, hehe. And the solution of course, is the Small Bust Adjustment. I haven't needed to do one for this top, but if you feel you need to, check here and here and here. Didn't find any great videos, so if you come across one, lemme know, I'll put it up!