Identify the joints and constraints between the various bodies. Joints can be real, such as that between a piston and its case, or virtual, such as that between two planets.
Model the degrees of freedom between bodies by connecting their frames through joints. You can further constrain these degrees of freedom through specialized constraints, such as those between gears. See Assemble a Gear Model for an example.
Specify the state targets of the various joints. You can specify the desired position and velocity of a joint at time zero. If the state targets are valid and compatible, the joints assemble in the states specified. See the Guide Assembly and Visualize Model section of Model a Closed-Loop Kinematic Chain for an example.
Update the block diagram. Examine the model visualization for assembly issues. Open the Simscape™ Variable Viewer or the Simscape Multibody™ Model Report to see if all state targets have been satisfied. See the Verify Model Assembly section of Model a Closed-Loop Kinematic Chain for an example.