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One approach is to find an existing URDF that has the same basic topology (possibly Turtlebot) and then modify it (in best case just updating the measurements of each piece). The model covers two aspects: the visible model which is used for Rviz visualization, and the physical model which is used for planning and to ensure collisions don;t occur. You don't need perfect 3D mesh designs for ROS moveit/planning - the meshes are just for visualization.

For a simple robot there are three main groups: base, arm, and gripper. I haven't modelled a base, but I believe, at a minimum, you will need the outside measurements and you will need to ensure your drive mechanism is modelled correctly - e.g. you will need to make sure the wheels are measured at the right location and are right size. You will need to ensure the right drive mechanism is modelled (tank-ackerman steering, or holonomic, etc).

For the arm, you will need to have the correct number of joints (e.g. Degrees of Freedom DOF), you will need to measure how long each link is between joints and you will need to measure how much each joint can rotate.

For the gripper, you will need to know whether it is a parallel gripper, a trapezoidal gripper or a single-sided gripper and you will need to measure the parts of the gripper.

Once you are done with the XACRO, you then convert that to URDF and then run MoveIt setup assistant to generate the SRDF file.