prework: If you need this part, then you will likely need to clean up the printed parts. Make sure the screws and belts go through the holes. It is not necessary for the nuts to fit in their slots.
1) Thread a small screw through the side of the base piece with a nut on the inside. Slide this onto the x-axis mounting point of the printrbot and tighten the screw to secure.
2) Thread a long screw through the smaller piece and put a nut on the other side. Have a small zip-tie handy for the next step.
3) Thread both ends of the x-axis belt through the holes in the sides of the base piece. Pull the belt through until it is tight, then slip the smaller piece between the belt ends with the end of the screw facing up. It should be as low as possible. Use a small zip-tie to secure the belt ends to the smaller piece.
4) Tighten the long screw to tension your belt. The end of it should slip into a hole in the base for stability.