First off start by buying a notebook and filling it with ideas and spells that you have made or other spells not by you.
Decorate it make it your own its your spell book not your notepad.
I would suggest starting it with your name your age your interests and then writing your ideas down. Try Writing about your interests for example.. If you were interested in Geokenisis And herbs and healing. Write A bit of that the most basic things come first then then the advanced things like main ingredients recipes.
By now you should have got well into your spell book.
Make a contents a date at the top. Number your pages!
I always get fairly annoyed with mine because sometimes you get writer block and you cant think of anything. So take a walk out a stroll do a bit of housework then come back to it when you feel like it.
DONT just go to a random website and copy everything in sight describe it in your own words.
Write in it DAILY.
Dont leave it alone you never know when you might need it!
Thats all the advice i can give you feel free to add on other people.