By this I mean, does it (for example) send a sizzling ball of thaumaturgical fire shooting through the air to burn a target? Does it call forth a great hand of clay from the ground to crush your enemies? Does it allow you to float through the air intangibly, like a cloud? In other words, what are the Special Effects of the spell when it is successfully cast?
The game mechanics involved in actually defining and costing the spell are completely unimportant when you are conceiving a spell; leave all that until the very end when everything else has been decided. There's nothing less inspiring than hearing a player say "I'll cast my generic plain-pack 8d6 Energy Blast Spell"; it does absolutely nothing to aid in the suspension of disbelief which is necessary for enjoyable roleplaying.
In other words, what hocus-pocus is involved in spellcasting? Does the spell go off immediately, or is there a delay? Does it need any particular material components? What are they? Are the spell components consumed by casting the spell? Does it require an incantation and/or magical gestures? What sort of gestures? What, if anything, happens if the spellcasting is interrupted or carried out incorrectly?
These considerations (and others) will determine some of the Advantages and Limitations which will later be applied to define the spell according to the game mechanics.
Do you read it and learn it from a spellbook? If so, how long does it take? Do you have to relearn the spell every time you cast it? Once learned, does it stay in your mind forever, or does the knowledge of the spell fade? How many times, or over what time period, can you cast the spell before you have to relearn it?
Is the power granted to you by a demon imprisoned within a globe of amber? Why is the demon being so generous? Do you have to reward or appease the demon? How? What happens if the demon gets free?
Unless you're playing an extremely unimaginative wizard, your spells are unlikely to have names like "Type VII Electrical Blast" or "Spell for Lifting Heavy Things". A bit of thought put into the name of the spell will do great things for defining the feel of your magic.
The last, and arguably the least important stage from a roleplaying perspective, is actually defining the spell in terms of Powers, Skills, Talents, Advantages and Limitations. It's a necessary evil, since you need to know the power level and what-not of the spell, but don't fall into the trap of trying to design spells around game mechanics. The game mechanics must be made to fit the conception, not vice-versa.