The most important part of spellcrafting is knowing what you want from your spell. Always know the result of the spell before you start creating it. Be specific about your desires. I find it best to write down exactly what you want, then go from there. Don't be afraid to let your spell evolve as you start your spellcrafting process. If you are specific about your spell, you will not recieve benefits (or disadvantages) that you were not expecting.
Spellcrafting is an art and a science. You must realize that every element you put into a spell has a reprecussion on the spell itself. Certain elements have different effect than others. A spell can be as simple as a chant or as complex as a multi-layered circle with quadruple summonings. It is up to you and the amount of energy you need for your spell to work.
Spells can be cast anywhere at anytime. There are times and places that tend to have more energy available for you to channel as well as certain times. Moon phase can have an impact on energy, as well as season, time of day etc. Numbers, colors, weather etc also have an effect on the energies around. However, it is best to do a spell when and where you feel most comfortable. Taking advantage of certain attributes around you can help or harm your spell.
In the end, how you craft a spell depends on you and what you are comfortable with. But, here are a few basics:
Spellcrafting is a tool like any other. For it to be successful you have to put into it what you desire out of it. You should also learn how to use it to its full advantage. You must believe in it and yourself for it to be successful. Spellscrafting will help you gain a depper understanding of yourself and those around you. You will become more in tune with various types of energy as you practice working with them. Like anything else, spellcrafting takes practice.