Study "Assembly.skp" or "Assembly.stl".
Print all pieces as shown in "Assembly.skp" or "Assembly.stl". For "Wing Left.stl" and "Wing Right.stl", I started printing in black, paused the printer at 3.6 mm, changed the filament to white, then resumed printing.
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.
Assemble as shown in "Assembly.skp" or "Assembly.stl".
To make a pull toy, slip a length of yarn into the lower front hole in "Chest.stl" then out through either side, tie a large knot, secure it with glue, then pull the yarn back in so that the knot is inside the hole.
Comment if you have questions.