While H.P.Love Crafts writings are fictional. I believe Robert1979 is referring to The Simon Necronomicon.
The second one, called the Necronomicon Spell Book if I'm not mistaken.
The Simon Necronomicon is not part of Lovecraft's lore or fictional universe. It is Based on (Loosely) Babylonian Mythology. You'd be casting spell by invoking The Gods/Demons/Spirits from There.
However before using this book there are somethings you should know that throws the accuracy into question.
For starters Though given to Simon by a "nameless priest" And Supposed to be true Babylonian spell book You can clearly see the influence of Lovecraft's works. Some parts of which are almost taken from LC almost word for word. I'd call it a Hodgepodge of Babylonian/Lovecraftian lore.
However Let us assume that the books are accurate enough for magic castings. There are some other issues that crop up. If you go by the first book, It tells you that this is not a beginners grimoire, The Author flat out says it at some point in his introduction while explaining the history of how they got hold of the manuscript, translated it etc.
The book assumes that the reader is both an experienced caster And is familiar with the subject matter.
The first book describes opening of the gates to access greater power and to communicate with the Gods that might grant it. To open the gates requires Incantations and Some rather specific tools to do so. like Silver plate, chalice with still warm goat blood under the full moon etc. Some spells and gates requiring hours to days to open.
The big emphasis is on the NOT A BEGINNER BOOK.
The second book Called Necronomicon Spellbook Is the first book made easy. All you need is a white peace of paper black pen and 2 white candles. Draw the symbol in a circle for the spell you wanted cast. Think about the result you are aiming for, light the candles, and Chant the incantation. Prefferable do it at night when the moon is waxing (but its not necessary if you need to cast it at other times) Continue to hold the thoughts of what you are trying to achieve. Blow out candles fold paper and don't show the paper to others. The Spells take between 15-40 minutes to cast.
The thing that raises some Eyebrows in the community about that is the Vast difference in preparation,requirements, tools and difficulty from the first book to the second.
However there are those that believe or have found that In some cases the source of the magic is irrelevant. If you have the will, talent and/or strength, the spells will work anyway.
The formula(incantation) is just a way to help you visualize and focus the energy to accomplice your goals.
So if it works for you by all means go a head with it. However odd are that these Necronomicons are about as accurate as H.P Lovecrafts. I think Darkbooks org has a free pdf of the two aforementioned books. *Not sure if they legally have it or not*