Can I make my own custom spell? And will it work?

Of course! Just like you can bake a cake without a recipe. For some people, guidelines are needed, for others they are a problem. However, if you don't want to ruin your spell (or your cake) you need to know what you're doing once you drop the cookbook. The main issues are safety (yours and others') and efficiency (know your gods and their symbols). Be especially careful if you invoke any kind of power, that you are not being played by something bigger than you. Always understand the full extent of consequences, be careful what you wish for. In my experience, a good custom spell is significantly more powerful than a bland template.