A thumbnail cannot carry the same step-by-step detail as a tutorial. Two carefully selected states can communicate a process more clearly than a collage of every stage.
Choose the informative transition
Pick the starting problem and the meaningful result, or the key action and its immediate effect. For a folding demonstration, an unfolded sheet and the finished form may be enough. Intermediate steps belong in the lesson.
Make the connection visible
Keep the material, angle, and scale consistent where possible. If the states look unrelated, a small directional cue may help, but avoid filling the image with arrows and labels before trying a clearer pair of frames.
Check the expectation
The chosen states should represent a transition the video actually teaches. Do not display an advanced final result when the upload stops halfway through the process. Review both images at the intended small size and remove decorative elements that obscure the change you want viewers to understand.