So I have been building for a little while on ROBLOX (before I took one giant break from ROBLOX in general because of school).
I have finally got back into ROBLOX and saw the new even and wanted to know how to build something cartoony, as all of my time on ROBLOX I never knew how to do it. Thanks.
Vibrant colors. Low detail. SmoothPlastic material. It’s more so known as a low poly look. That’s it, really
I highly suggest using blender to build if you’re thinking about doing low-poly, it’s hard to learn at first, but will make building a lot easier since most cartoony builds have lots of curves and triangles.
Most cartoony styles of buildings use the plastic material. They usually look something like this:
Now this example is not very cartoony, however, the cartoony style is somewhat related to the low-poly style. Hope this helps! 
Practice and use inspiration. You should learn how to do it pretty easily, since most cartoony type of builds are low poly. First I recommend using pictures of cartoony builds online and try copying or using them as inspiration in Roblox Studios. After you get a hang of that, try making your own (without copying). Just like someone said above, Blender can help you with making a building cartoony. You do not necessarily need Blender to make it though.
Topics related to this question have been discussed before in previous topics, have you done a quick research around and see if those threads answer your question on how to accomplish this. This could be done with alternative software put it isn’t required if your wanting to make them using only ‘Roblox Studio’.
As for the question; The cartoony style, is mostly achieved by using bright/ vibrant colors and use of the plastic material and a good lighting source to make the colors reflect those vibrant looks. It’s not to complicated to create, I’m sure if you start simple you would accomplish a quality build with the style.
