I think a possible way to make your own spells is to go off prior knowledge. You can change spells you already know. Most spells are open for interpretation. To create your own spell you can just do what feels right create your own chant say for a blessing create a chant and hold the object your want to bless. You are the one creating the spell therefore you can make it however you like. Try to look at why all ready made spells are the way they are and then use logic and imagination to create your own good luck hope this helps.