For karts, you will really need a concrete idea in order to avoid spending precious time revising. Unlike with tracks, it's difficult to make small adjustments because karts are inorganic in shape and small in size.
First, choose a character. For SuperTuxKart, at least core STK, it should be a mascot related to some open-source software. There are plenty of them. Wikipedia has a good list of mascots which are actually mostly (if not all) open-source-related.
Once you have a character, it's necessary to think about how it will be oriented in the kart. Draw the character in a driving position, think about different poses the character could have, etc.
Decide what the kart should like like, then make a rough sketch without your character. You might have to go through several revisions, making sure you've covered several different angles of the kart. When you're satisfied that the kart looks good, do a final drawing with your character inside the kart. Remember that concept art doesn't have to be perfect, but it is important to get a feel for how your kart will turn out. You'll want to refer to it as you build your 3D model.