First thing you need to do is draw something on Inkscape. I am not an artist so I will just draw a star here…
Inkscape also offer the ability to draw 3D boxes with manipulative dimensions but I suggest you use 2D drawings as something goes wrong with the 3D import to TinkerCAD.
After you finish you drawing you got to save the file as SVG, otherwise you won’t be able to import.
Now you can visit tinkercad.com (use a browser that supports WebGL) and create a new account for free in just a few seconds. This will grant you access to the drawing tools and the very helpful and friendly game-like tutorials. I will assume that you took the tutorials to get the idea of how the editor works and proceed to importing the file from Inkscape.
You open the editor to create a new drawing, and on the top right you can see the option to import. You press this button and then choose your drawing .svg file from your hard disk and then press the import button. Remember to adjust the scale percentage before importing if the imported object is too big (it probably will be).
You can then adjust the height and the thickness of the object, add more objects, combine them, resize them, mix the shapes and meet the desired result. After all that you can save your project on-line, print it using one of the services you can see in the following screenshot, or just download it in .stl, .obj, x3d, or vrml format.
Of course, this is not the most advanced and sophisticated way to draw 3d objects but it certainly is a great way to do simple things in a very easy and friendly way!