What to Know:

You need more than belief to make a spell work. You need like energy, imagination, patience, etc.. So make sure you try to achieve those things for better spell casting. 

Try other Spells:

Since spells require confidence, energy, etc, you can try spells to get you these like you can do energy spells, confidence spells, etc to help you achieve the spell

Forget:

What I recommend is that you try and forget the spell. I saw and it said it will help you in spell casting. Once you cast a spell, don't put a lot of thought into it.

Focus:

While doing a spell, try to focus into it. Like keep thinking of what you want from the spell, what you want the spell to do, things that deal with the spell etc.. Like when doing a mermaid spell, focus on what deals with mermaids like water, fishes, etc..

Confidence:

This is one of the big problems with spell casting. People get too scared to try them and it ends up with the spell not working for them. Try to overcome that. Spells is about confidence too. Confidence also get spells to work. Try not to be scared, get ready for the effects. You can also try the confidence spell on my channel.

Belief:

This is the biggest problem with spell casting. A persons belief can go down by giving up, other people, etc.. Believe what you believe in. While doing the spell, think It will work I can do it etc.. Try to put more positive thoughts into it. Do not put negative thoughts. You can also say I believe in your head while chanting the spell

Ingredients:

Some spells require ingredients. Some require a lot, some require a few, and some don't require anything at all. When getting ready for the spell, when it says the items you need for it. Try to find some that goes along with the spell. Like for a fairy spell, when it says items that represent the earth. Try to find some leaves, flowers, etc..