STEP 1: Take a screenshot of the knyttstories screen you want as a cutscene. How to do so is covered in the [FAQ]
STEP 2: Paste the image into an image editing program and crop it down to size.
STEP 3: Save the image as Scene1.png in ...Knytt Stories\Worlds\[your level]\Cutscenes\[name of sutscene].
Alternately, you can shift into the screen and shift out again. How to make shift cutscenes, or shifts for that matter could probably be found out with a simple [search].