Attempting to morph an idea in your head into a finished crochet item is design. The sheer amount of details can be overwhelming! What type of yarn would work best? What color? What weight? What fiber content? What hook size? What stitch pattern? Then, of course, comes the writing of the pattern.
I like to jot down notes that will eventually be turned into a pattern as I work. This is always done in pencil so I can change it easily when I rip out that last row because it just doesn't look right. I've read how some people will make the item first, then rip it out row by row, writing the pattern in reverse, counting stitches, and measuring how many yards they've used. This method allows you to follow your own pattern to re-make the item, editing as you go.
Modifying a pattern is much easier and generally less time consuming. Someone else has already designed the item, and you are simply making a few changes here and there. This can be anything from adding a lace border to adding sleeves to a vest pattern to make the perfect sweater. Or doing the math to make it a size or two larger than the designer wrote. Changing the hook size and yarn can turn a fitted sweater into an over sized one.
It's important to know the difference between designing and modifying when you want to sell your work. If I designed it, I do not hesitate to put my label on it. If I modified it, I will put "based on the pattern by so-and-so in such-and-such publication". Always give credit to the original designer - it's the right thing to do.