Oh gosh, get ready to be underwhelmed with my answer- because I know if I was in your shoes I would be a bit disappointed.
Spells are not a universal thing. Magic has existed all over the world in different forms for as long as humans have existed, or at least as far back as Anthropologists have discovered. There are SO many different forms and paths of magic, so stating one criteria for magic is nearly impossible.
So when people ask me how to create a spell, I always recommend looking internally instead of externally. Why do you want to do the spell? What things remind you of your purpose? What are the pros and cons of casting this spell? ect.
For example, pretend I am a new witch and I want to cast a curse on someone who hurt me. First of all I would try labeling the spell with one word to keep it from becoming an "everything" spell (meaning you do one spell for basically everything you can think of, which never works in my experience). For me this would be a revenge spell. Now time to think of things that remind me of revenge. I was raise Christian so nails could represent betrayal and revenge. Maybe I would add rusty nails to my spell. Then I would just keep adding things to represent certain ideas until I personally felt it was enough.
Spells are very personal. It really is up to you on how to design one. Just make sure that the items you use are personal to you and you speak from your desires.
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.