You have many options for how you want to actually record your information into your grimoire. My only recommendation here is to test out whatever writing utensil on a scratch sheet of paper before writing in your grimoire. You don’t want the ink to bleed through to other pages or smear as soon as you close the book.
Pens and pencils (convenient and reliable)
Crayons, colored pencils, or markers (good for color coordinating, organizing, and using color magic)
Ink and quill (a messier and more expensive option, but can have beautiful and interesting effects)
Typing out the information, printing it out, and pasting it into your book
Some witches like to keep the information in their grimoires secret by writing in code. You can try using a numerical system (such as A=1, B=2, etc), or create your own special code. You can try using “the Theban alphabet” which uses symbols that correspond with our alphabet. You might want to only use a code for your most secret, personal spells, such as a special incantation or a secret ingredient.
You might enjoy adding some witchy touches to your grimoire such as pressing flowers, herbs, incense, or powder in between the pages, rubbing a bit of oil on the corner of a page, or spritzing it with perfume or a meaningful scent. Again, anything goes. If you want to try some, go right ahead. Have fun, and enjoy the process of creating and decorating your special book of magic.