How does one create a spell: First off, figure out what your intent is, is it a banishing spell, a love spell, a sleeping spell, or maybe even a healing spell? This is probably the easiest part of making a spell. Secondly figure out what you want to use in your spell, are you good with crystals or gems, herbs, incense, sigils, do you worship any dieties, what types of magic are you familiar with that you could use? Also is there a specific time you can use this spell, does the moon need to be out, can it only be done on a tuseday? What does the caster need to cast it? See how it gets a bit more complicated?
Is there any certian criteria need to be met in order to create a spell: I would recomend cleansing your working area, casting a circle and then invoking ones gods before every spell and then thanking your gods and closing a circle after but that's up to you. Other than that I think you just need intention and meditation, some spells are simpley incantations.
How can a spell be strong: Four words - meditation, intent, energy and ingredients.
Is there anything one must do to ensure the spell works: Belief is a big part of it, if you think it's not going to work going in then the wrong energies could be sent out and just flub the spell instantly. The more positive energies and meditation you put into casting the spell the more likely it is to work.
I hope this helps.