As it was specified in the page of presentation of the assembly constraints, it is essential to immobilize a part (or a sub-assembly) in the geometric reference system so that the other objects are positioned on it and not the reverse, under penalty of seeing the positioning of the whole very fanciful for future projections.

 Select in the tree any constraint element of the object to fix
defined in the preparation phase, then click on the the Locked constraint   .

Note :
It is also possible for simple examples to select entities directly on the 3D view. These will automatically appear in the "Elements" group of the assembly container.

A recap video will be available at the end of this cycle.