In this step, you'll repeat Step 13. But, there are some changes you'll need to make, as shown in Pic. 1-3. The red rods should be yellow rods, and the red connectors should be yellow connectors.
Pic. 1- This is the bottom cart. Notice the differences between it and the top cart. (yellow connectors and yellow rods).
Pic. 2- Those red rods will be yellow rods (refer to Pic. 1).
Pic. 3- The red connectors should be yellow connectors (refer to Pic. 1).
Pic. 4- After the bottom cart in Pic. 1 is done, build these.
Pic. 5- Attach them to the bottom cart.