Magick is a very personal thing yet, it is also a literal ritual thing. Depending on what you are doing and attempting to accomplish decides between the two. If all you are wanting is some private personal desire such as cast a spell then Id say take from established practices or others spells and then make it your own. Like if you use a poppet id say to fill it full of the mentioned items and also add your own items that you feel at to what you want. then Make your own spell, mantra and or chant. For things like this the more personal you make it the better for its success. for you have then put into it what is meaningful to you thus attaching and focus your energy and will in to the workings. Such coping what someone else has done makes it very hard to emotionally attach to the working because its no longer personal for you which lessens the chance of success.
Now if you are attempting a ritual such as what it sounds like the creater of this thread is doing, following what is said in the necronomicon would be adviced. I can't say much towards what he or she is doing because ive never read the necronomicon. But if the sacrice is herbs or something of that nature, id stick with it, but if you cant find it then substitute with something with the same characteristics or species. Most rituals such as invoking forces or dieties its best to follow whats writen as it puts one in the mindset of what they are doing and may have important keys to accomplishing what is being attempted, but that does not mean you can not add if you know of a another practice that deals with that same issue and you feel will add to the success of whats being done. Like for example, I could be doing something with satanic magick but will add things from voodoo into it. Both practices that im working with would be aimed towards the same goal and to me adds more power to what im doing.