Casting a spell you have written depends on your mood when you write it. Casting a spell that someone else made, only the person who made it knows. The most important thing to know is that you must be able to visualize what you want to happen and the outcome of that.
I start by meditation, clearing my mind and pulling in the energy needed for the works. The thing is to keep the intentions in mind, and concentrate on the purpose. Using others spells is nice, but writing your own makes it more personal to you. That's the basis
Too be honest, it really doesn't matter who's spell you use. But you should know which spells are real and which spells are fake.
Its better to write your own spell because you have more chance of it working. Things is suggest you do before casting.
Meditation(set an intention)
Preparation for the spells(candles, what time you're going to cast it)
Then when you feel ready and prepared get read to vast your spell, but make sure you visualise the outcome.