Most importantly, you need an autograph book! They are sold in every gift shop on Disney property. There are simple books filled with blank pages. They hold the most autographs. Or there are photo/autograph combo books. These are the ones I like! One side has a blank page for signing, the other a clear sleeve for you to place a photo of you and the character. These books do hold less autographs though. There are lots of other options for autograph books, if you don't want one from Disney. There are tons of websites that sell them. There are even hand-made ones on sites like etsy. Or a fun notepad works, too. If you want to get creative, make your own!!
Next, make sure you have a signing implement on hand. Something thick works best, like a Sharpie or fat pen. Characters sometimes have a hard time holding small pens, pencils, or markers.
After you've waited patiently in line and it's your turn, have your book open to the page you'd like the character to sign on and hand them your pen or Sharpie. Face characters (like princesses, who do not have a full mask or "head") will often ask your name and chat while they sign. It's a fun experience. They always do autographs first, then pose for photos.
An FYI...certain characters do not sign autographs because they are physically unable, like Mike and Sully, Buzz Lightyear, Baymax, and Beast.
Young kids, maybe up to age 4, don't really "get" what's going on with autographs. I suggest waiting until they are older and really understand. My daughter had no interest until she was five. She noticed other kids getting autographs and just had to do it too! Not to say some younger kids won't be interested, but honestly, if they are real little, it might be better just to focus on them meeting and interacting with the characters.
