Map the construction
Identify fold lines, cut lines, and joining tabs before making the first cut. Dry-fit neighbouring surfaces to see which joins become inaccessible later. Working out the sequence prevents a carefully drawn model from becoming difficult to assemble.
Design around a silhouette
Use a few large volumes to establish the robot's character. Compare the shape from front and side before adding small features. Feet and joints need to support the object as well as suggest movement.
Make a test version
Build a rough prototype with ordinary paper and revise awkward tabs or unsupported pieces. Choose cutting tools appropriate to the maker's age and experience, and follow their safety instructions.