I'll repeat everyone elses explanation:
Lay the backing down on a flat surface tape it with little pieces of tape top center, bottom center, right center, left center, then corners, then working from the center out - continue flattening but not pulling, in between those tapes and add a few more.
Then add the batting flattening again from the center out (no taping). Now add the top again flattening from the center out.
Now pin using quilters safety pins (they have the perfect bend to get them back up for closure)again from the center out.
The batting - although a nice fluffy polyester seems cute and fluffy they're really not really the best for little ones. Maybe after 4 or 5 yrs old. I would stick with a warm and natural or something like that. It has a lower loft but still comes out really nice. Don't quilt it too close or it will flatten out too much and result in a stiff feeling.
Hope that helps and welcome from Spring, Tx