Each move forward through the 6 Steps should make the follow-up activity a bit of a challenge, but not so much that the child struggles. 

You can use this printable Set of Indicators of Readiness  showing what a child should be able to do before moving on. You also have this printable chart showing the  Skills and Concepts at Each Step  to see the what the child is accomplishing at each of the Steps.

But if you’re not quite sure, wait. For we want the child’s work to be enjoyable and give them confidence.

I also recommend you read these two pages, How and Why a Natural Approach Works and Curriculum Nature’s Way. These will give you an in-depth understanding of the reasoning behind this natural approach, meant to help you make decisions.

But my primary recommendation always is to think back to what both of you did when you were helping your child learn to speak. For while we’re more overt and intentional about it with print, the process is very similar:

You model, the child absorbs and practices on their own.

You wait patiently, letting them proceed, without pressuring them in any way.