As crochet is an artisan skill with some level of popularity, it is possible to make a little money with it. It is NOT a get rich quick type of deal, however. The upfront investment is yarn and a whole lot of time.
People who sell at craft fairs must create their inventory. Using a hat as an example, let's say that one hat takes 4 hours and one skein to make. To have 10 hats on hand to sell, 40 hours and 10 skeins of yarn have to be expended. Obviously, you would need more than those 10 hats to fill up your table, but you get the idea. The day of the fair can be a profitable one or not, depending on attendance and how many people purchase items from you.
Designing for magazines takes longer - sometimes, much longer. Although I've never worked on the editorial side of things, I've spoken to many designers and emailed publishers. As a designer, you come up with an idea, sketch it, swatch it, gauge it, notate it enough so you can make it on demand, and for fashion submission, draw a schematic with sizing. This process can take days or even weeks to complete. It can take months, sometimes even over a year, for an editor to contact you. They may hold on to it for a future issue.
About 7 or 8 years ago, I had submitted a laptop case (finished product, no sketch) to a crochet magazine that now no longer exists. Imagine my surprise when I received it back 20 months later with a "thanks but no thanks" note! By that time, the Lion Brand yarn I had used had been discontinued!!
The standard amount of time for a book deal, I am told, is about a year's worth of work. Booklets take a little less time, perhaps, but with either one a proposal must be written and accepted, examples of your work have to be submitted, several editorial approvals must be given....lots of administrative time in addition to crochet time.
Self-published designs still require you to go through the design process. Add to that making the finished product, photographing it, writing up the pattern, editing, and formatting. Once all of that is done, you can put your pattern up for sale on Etsy, Ravelry, or your own website.
Contests are a little different, in that you may or may not have to provide the pattern for the item you made. County fairs sometimes award cash prizes, as do several contests online, such as Herrschner's and Mary Maxim.
Although it is possible to make money from crochet, every item, every submission, every design is a gamble. You must truly love your craft and stick with it to profit.