Summary

https://youtu.be/ypQx8DEk7VA
This thing is a more simple setup, compared to https://www.thingiverse.com/thing:771004
Gear wheels are included and attached to a frame.
https://youtu.be/oOeriEbkMsY
For the miniature geared dc motor, a 9 volt battery is fixed with a tie wrap or a piece of double sided adhesive foam tape..
https://youtu.be/-gS8GaNyc9I
You might need to rotate the body to the normal position before printing. It is a good idea to trim the shaft openings in the frame after printing with a 4 mm drill.
The square shafts are driven gently into one of the leg wheels with a light hammer. Only gentle force should be applied. If a shaft breaks, remove it with an M3 bolt in the other side of the shaft opening and use a hammer to remove the broken part. When installing the wheels at the other side, the leg bolts should be opposite. Depending on the tolerances of your printer, it might be necessary to remove some material from the shaft ends using sandpaper, until they slip into position with reasonable friction. Eventually use a rotary tool to rotate the shaft and clamp sandpaper to the rotating end, applying pressure with your fingers. The legs are secured by a lock nut. The on/off switch goes into the tail. If your buggy walks backward, simply reverse the motor wires. A drop of oil on the moving parts reduces friction. The shell is fixed with two M3x50 bolts and separated from the frame with two printed spacers. If the legs are not exactly synchronized, buggy gets a funny wobly walk as demonstrated in the movie of the test walk.