Here are some steps in learning to create your own crochet patterns.
1) Crochet a ton of patterns! Let's just say you want to be able to create your own hat patterns. The first step is to make so many different hats that you start to get a feel for the general guidelines they follow. In the case of hats, for example, you always start with some type of ring. Generally you then work somewhere between 6 and 10 single crochets (sc) into your ring. From there you gradually increase until you reach the desired circumference, at which point you stop increasing and just do one stitch for each stitch around.
I'm going to continue using hats as my example, but remember, the idea holds true for all things crochet (baby booties, newborn cocoons, flowers, etc.)
2) Think of a specific design you want to create and start experimenting. Let's say you want to start with just a basic beanie like this one:
You'll need to decide what size hook you want to work with (how big do you want the holes?) and what weight of yarn you want to use (how chunky do you want it to look?).
Then just jump in! Start making a hat. If it doesn't turn out right, just take it out and try again (and again, and again).
You will want to increase until the circumference of your hat matches the standard head size for the size/age you are trying to make. You can find standard head sizes here. Then continue down a stitch for a stitch as mentioned above until your hat is the length you want.
Some other products aren't as easy to decipher size. You may be able to find size guidelines by consulting google, but often times you may need an actual model to try them out on.
3) Once you get your product just how you want it, count stitches and rows on your product to remind you exactly what you did and record your new pattern in a Word document.
That's it, you've created a pattern! Don't be deceived by the fact that there are only three steps. Designing a pattern takes time and patience!
Once you have a pattern of your own created it is much easier to create more because you can just use your pattern and change it to provide many different varieties. I have created so many hats and each one gets easier to create as I have more experience.
(and these are just a few of the many!)
Once you get hats down, you can move on to more complicated things like cute photo props:
Important reminder: When you set out to make your own pattern you should never be looking at or consulting (even just a little bit) someone else's pattern. Once you have created a pattern or two of your own it is perfectly fine to consult, change, and combine your own patterns however you would like, but never with someone else's pattern. Unfortunately, not everyone realizes that combining multiple patterns that are not your own and/or taking pieces from someone else's pattern is still copyright infringement. So my point is, be careful and courteous when creating patterns.