There is many ways to create a spell, it depends on how in depth you want to go. Some spells can be simple, which is easy and still powerful, while others can be a little deeper or more difficult.
Theres a lot of things you can put into a spell to make it more powerful, like herbs, crystals, colored candles, corresponding, or certain Deities. The options are nearly endless but always do your research on the item before bringing it into the spell and you don't need to go crazy adding all these herbs and crystals and such, just use whats needed.
Here are some things that I use to help me when designing a spell. You can take it or leave it, everything works differently for different people.
From Bill Whitcomb's book, "The Magician's Companion", He gives a list of five essentials to spell casting, spell work, etc. Its a pretty good reference for beginners. Here it is:
"1. Intention -- You must know what you will and be unwavering in your determination.
2. Preparation -- You must be prepared and possess all needed items before beginning a rite.
3. Purification -- You must avoid anything that could cause a mental or physical disturbance in you before or during a ritual
4. The Circle -- You must have a secure place of working where a circle can be cast
5. Consecration -- All tools and ritual items should be consecrated to their magickal functions."
In Silver RavenWolf's book, "Solitary Witch," She also gives tips on designing a spell. Here is what she wrote:
"--Each spell should have a statement of intent.
--Most spell work should be done in the magickal circle
--All items you use should correspond to each other, it all should be balanced.
--Always work for the "best possible outcome" no matter what the situation or problem
--Research all of the parts of the spell, know what deity, colors, herbs, crystals, etc match each other through energy patterns before you mix them together.
--Try to remain focused throughout the spellwork.
--Don't do spellwork if you are angry or stressed. Always remember to ground, center and breathe
--Be honest and honorable in every magickal work, sometimes a spell isn't needed - a attitude adjustment is whats necesary."
Take your time in writing a spell, the more you put into it the better it'll be. Everyone writes spells differently, so do whatever works for you.