Take a look at this card and you will see what I mean:

Seriously, is that not SO pretty? Here's a closer look at some of the flowers so you can really see how much you can “goof” and have it look so gorgeous:
You see how not every flower petal or leaf has the same amount of ink? So what? See how some are light and some are dark? So what! See how some are outside the lines? So what! It's all BEAUTIFUL! Trust me, your ‘ta-da' moment comes quickly since this doesn't take much time at all. My best tip – go for it, don't try. Just let what happens happen. No matter what you do, with this style of illustration and some watercoloring you are good to go!
I encourage you to pull out your ink pads, your Aqua Painters and a great neutral ink pad and go for it! Once you get hooked on making handmade watercolor cards you won't want to stop and since it doesn't have to be “perfect” you will LOVE it!