Spells are spells, there not a true way, or the right way. Of course you can write your own spells, I mean, spells had to evolve from someone, at sometime.
At first, I suggest making a few changes. Not a whole rewritten spell, but maybe a miner spell.
I remember when I first started, I could not
get anything to work. No matter What rating the spell was! So I suggest, learning the basics. Grounding, centering, meditation, and some correspondence 's. I
am now, 4 years into my studies. I can rhyme no problem, and dealing with my love of substances. I can incorporate many things.
If your a beginner in spells, I suggest only doing a spell it two a day. Spells do take energy, time, and will. It want happen overnight, unless it is meant to be. It will usually take a week, it two. Good luck