Rahul, I'm a Solidworks user, however, regarding your question...are you asking what is Top Down Assembly and Skeleton structure modeling or are you asking how to do in context modelling with ProE i.e. which tools to use and where to find them etc...
in assembly mode>create component> skeleton model>copy from existing:-ok
skeleton model has now been added to assembly
In model tree that skeleton model has been placed at the top of the list, even before assembly features
now open up the skeleton model separately to add datum geometry to this model
Skeleton model only contain profile of assembly or outline that you want to capture,do your thingy here (ie sketch outline)
make separate sketches for component for convenience.
save and close the skeleton
again in assembly create component>part>solid>enter whatever name:-ok>copy from existing
repeat this same process to create the remaining components (equal to number of sketches you made in skeleton)
Insert>Shared Data >Publish Geometry>click on sketch from skeleton whatever you want to publish.
open first part from model tree in assembly>insert>shared data>copy geometry>do your thingy
save and close repeat same for other individual sub assembly or parts