Writing a new spell can be as simple or creative as you want it to be because the important part is the focus, like you said. Now, there are certain things you can use in the spell to increase its power. Herbs, crystals, offerings to deities (depending on your pranch of paganism). Some spells have no words at all, just thought projection. Some spells have lengthy written out lyrics that may or may not rhyme. It really depends on what works for the caster.
I don't personally use the spells on this site as they are written. I will take a spell I find and add my own touches to it to make it unique. I have also written some spells from scratch myself. So be creative. The sky is the limit.