You don’t necessarily have to use this specific bit, screw and nut size; however, the idea is to attach these two pieces together so you'll need to make sure the holes you choose to make are compatible with your mounting hardware and each other.
Make sure the screw and nut are not to tight, you want the joint it creates to move freely.
The body movement mechanism should end up looking like this:
Now that you have your two servo motion mechanisms built, it is time to mount them to the doll. Again, servo positioning is very important to get your desired motion. The combination of what position you mounted the servo horn into its spline and the actual position of the servo when you glue into the doll’s body makes a huge difference in the range of motion you’re going to get.
Now you are ready to mount the servo on your platform of choice. You can mount the servo to a wall, bookshelf, waste basket etc. I chose to mount it onto a box. Remember the doll's front needs to face the object you mount the servo to so it can "peek" out from behind it. If it's an object you don't want to potentially ruin with hot glue, you might need to find alternative ways to mount the servo or use a different item.