You need to learn all associations in magic .Such what kind of herbs are used for what .What kind of crystals are used for what .Same goes for colors and moon phases and even planetary hours .To write your own spell does require vast ammount of knowledge in the craft .Also some knowledge on essential oils ,incenses, tools and if you are good in writing poetry you should create easy the spell itself .In short it is not that easy task that it looks like and often you need to get inspired by things too in same way as you need to get inspired to write short stories or poetry otherwise you may find yourself staring into empty sheet and wondering what on the earth you should write down .Spell uselly have ingredients section ,then method of their use and at some point you have to write down the spell itself ,so you have to make sure you have all of the sections filled before you end up the writing process .Also i tend to do first draft and then retink and recheck everything if i did not put the wrong ingredients for the task ,or if i need to work more in the spell and its rhymes .
Have at least one book on associations in magic handy till you remember them all .
Have a separate notebook with all words which are rhyming well so it wil be easy for you to construct the spell .
As in everything else experience and trying make perfection ;)
Good luck .