It isn't just the construction of the spell that determines whether or not a spell will work. Magic in general works on sympathetic action and symbolism, so the person casting the spell needs to use a method that allows them to personally connect to the process.
Anyone can throw together a few items and a set of instructions for using them and call it effective, because the symbols and actions they choose are meaningful to them. But not everyone makes the same associations. So for one person a spell of protection might be as simple as ringing a bell and letting the tones clear the air. For another it might require saying an affirmation or prayer. For still more people it could be a call to their guides, or a visualization of being filled with positive energy, or drawing a circle in chalk or salt or stones. All of these different symbolic ideas represent protection, but only one or two might work for you as a person because it can't just 'say' protection. it has to -mean- protection for you personally.
If you want to get to a point where you can work effective magic, then be prepared to practice a few things on the way. Patience, introspection, visualization, symbolism, sometimes religious studies and history, philosophy, and... well... any other knowledge a person might get the opportunity to gain regarding the path of magic/faith they intend to follow. Like many things in life magic is a journey, not a switch.
A major part of that journey is coming to understand how magic works, and what it will and won't accomplish for you.
For example, you asked for fire. This is a pretty broad concept, so a little more detail would have been helpful like how you are wanting to apply or work with fire... but I'll use what I have.
When it comes to fire there are many things magic can not do. for example conjuring fire, throwing fire, shaping fire, being immune to fire, etc. the accepted guiding rule is that magic works with nature as a complimentary force. It doesn't allow one to work outside of nature or to bypass physical laws.
However one can use the presence of fire as a symbol for a variety of things. being one of the basic magical elements, one can emotionally and symbolically connect with the idea of fire as a visible and tangible source of energy to draw from and work with. Fire is also a transformer, as it is a visible reaction of the conversion of matter into energy and light. it can also be used as a focal point for meditation and scrying by observing a small candle flame and using it as a catalyst for entering a relaxed and focused state. Flame can be used to burn notes, written spells, and other things as a symbolic gesture of releasing a physical thing into the spiritual realms. Fire can also be used as a beacon or guiding light to draw desired entities or other beings to your presence. it is also a force of safety as the light it casts chases away darkness and shadow, and allows one to physically and metaphorically find their way.
The same goes for any other idea as well. Spiritual shielding won't stop fists, or bullets, or even snowballs. but it will help protect you from negative emotional influences. By reaffirming a sense of identity and control over yourself it allows you to avoid dwelling on the negativity of others, or the influences of potential spiritual attacks (like from supernatural oppression, etc).
However if one wanted a way to use magic to avoid physical harm, then one way to achieve some measure of success is to work on using magic as a method of practice for personal awareness and being in the present moment during everyday life. By seeing the potential for harm one can avoid bad situations, or otherwise be alert to their surroundings and actively preventative in coming to a place of risk to begin with. Basically, the best way to stop a bullet from hitting you is to not go standing in front of a gun. Or, to avoid getting punched don't go to a bar looking for a fight. take your pick of analogy. ^_^
