Yes very broad topic.
For a design, one has to begin with a concept, which can range from that featured in the wiki article from Borg, where the system is attached next to the human body, to something such as portrayed in the movies, such as the assisted freight forklift system used by Sigourney Weaver in Alien, where the user can just enter into the machine and easily activate; or as elaborate as that from the movie Ativar. You will need to be familiar with several disciplines, as shown in the Wiki.
Start.
From the concept, one can determine possible dimensions of the machine and its components, and possible range of movements.
From there, forces, torques, stresses and strains at the various joints and members.
Next, a selection of the components, which would include the actuators, sensors, power supply, materials for the members, etc.
Keep a running bill of materials into which one adds the component, dimensions, material and amount, cost of fabrication, time for assembly,etc
Additional list items ...
Go to start if something needs adjustment, expect to do that several times.
Once done, select a paint color, logo. ( you could do this at just about any time but it will change )
End:
Present it to the world.
And charge a lot.
Don't expect to do it all in one weekend.