Designing an original crocheted piece is sometimes a great deal of hard work. If you don't know where to start or think you have no idea how to even try, think about these things: have you ever modified a piece to get it closer to what you wanted? Even something as simple as changing the color of the yarn is a step in the right direction (and I'm 99.999% sure we've all done that!). The first step to designing (in my opinion) is modifying existing patterns to suit your needs or tastes. This post will be about adjusting patterns.
1. Besides simply changing the color of the yarn to get what you want, you can change the size of the finished product. The easiest way to do this is to use a different hook size. Follow the pattern as it is written but use a smaller or larger hook.
2. Just as easily as changing the hook size is changing the yarn type. Most of the products I make use worsted weight yarn. I like how it feels on my hook and I'm used to it. If I use a thinner yarn or a thicker yarn, that automatically changes the finished size.
3. Another way to change the size of a finished product is to change the number of repeats. Most patterns are repetitive and often the designer tells you the pattern is "multiples of *number of stitches* plus *number of stitches*" for example: multiple of 7 plus 2. So if your pattern calls for 65 stitches across you can change it to a smaller size by multiplying 7 x say, 5 = 35 plus 2 for 37. This would make the finished product about half the length. Of course, you don't need to change it that drastically!
4. Stitch length is another variable in patterns that can be easily changed. Try using a taller stitch sometimes, a double crochet instead of a single crochet perhaps. Or do the opposite. Be a bit more discerning with this change though, because it can change the entire look of a finished product. You'll need to make sure this is a change that you can appreciate. Then again, making changes is a huge part of the designing process! If you don't like it, rip it out and try again.
5. Replacing an element with a different method. If you're making a wearable piece that calls for an element in a method you don't care for (a sweater with seamed sleeves instead of seamless or a slipper sock with a gusseted heel instead of a heel flap) you can work the pattern as it is and replace one method for another when it gets to that part of the pattern.
Once you are comfortable making these changes, you can start modifying existing patterns more and more with a combination of these suggestions. If you make enough changes, you've actually created a new, unique piece and can call yourself a designer after all! Do this enough times and you can start from scratch.