Study "Assembly.skp" or "Assembly.stl".
Print and assemble all pieces as shown in "Assembly.skp" or "Assembly.stl".
Test fit and trim, file, sand, etc. all parts as necessary for smooth movement of moving surfaces, and tight fit for non moving surfaces. Depending on the colors you chose and your printer settings, more or less trimming, filing and/or sanding will be required.
Place wide rubber bands around the circumference of each wheel for traction.
Slip the yarn through the hole in "Head.stl" down through the dome, tie a large not, secure it with glue, then pull the yarn back such that the knot is hidden within the dome.
Assemble as shown in "Assembly.skp" or "Assembly.stl".
After assembly of the wheels to the main axel, a small dot of thick cyanoacrylate is applied between "Wheel Section 1.stl" and "Axle Main.stl".
Comment if you have questions.