Although you can make a skid-bot with the Educational Edition of the EV3, the caster allows you even greater maneuverability because there is less friction. Having said that, the skid will actually respond better to bumps than the caster if your terrain is not perfectly flat. But I am interested in a robot that can be maneuvered with precision, so I am going to have us build a robot with the caster. Let's start out by gathering the parts seen in the Bill of Materials that is shown in the following screenshot:

Start out by flipping over our chassis:

In our caster submodel, we will build an assembly to attach the caster to the chassis:

Next, we will attach the caster assembly to the chassis:

We now need axles to attach the drive wheels to the large motors. I am using the 8-mod stopped axles. The bushings are merely to provide extra space so that, as far as possible, the axles do not stick out of the robot and to prevent slippage.

Now add bushings to the outside...