How to create Omniwheel base URDF

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Hello all, for my project i had to create a robotic base for manipulators equipped with the omniwheels (the same of the youbot base for example), i create the cad in Adobe Inventor and extract the stl file. Now I want to create the Urdf then the xacro for using it in rviz and gazebo. I read some tutorials but is not really clear how i have to do for creating a model working in ros. is some one who can help and suggest me some good tricks ?? thanks in advance