-------------------------------------------------------------

1. Apply mirror modifier in object mode

(bones have to be assigned to actual vertices, not a mirror)

-------------------------------------------------------------

2. Add armature

Click on the joint ball - extrude with E

-------------------------------------------------------------

3. Select and Duplicate bone section

Mirror duplicated section

align by pressing x, y, z + snap 

-------------------------------------------------------------

4. Display bone names:

armature data -> viewport display

[x] Names

-------------------------------------------------------------

5. Connect duplicated bone to the original joint

Bone properties - Relations:  [x] Connected

-------------------------------------------------------------

6. Parent mesh to armature

select mesh + armature (Ctrl)

With cursor on main window - in object mode: hit Ctrl-P

alternative 1: With Empty Groups

alternative 2: Automatic Weights

-------------------------------------------------------------

(in order to clear parent: Alt-P with mesh selected)

-------------------------------------------------------------

7. Assign vertex to bone (With Empty Groups)

Select mesh in object mode

open the Data section

enter edit mode with TAB - select and assign vertices to bone groups

-------------------------------------------------------------

8. In object mode -> select Armature

enter Pose Mode (not edit or object)

-------------------------------------------------------------

9. Open Dope Sheet in another window

change mode to: Action Editor -> click new

(several animations for one object)

-------------------------------------------------------------

10. Rotate bones in Pose Mode to animate

select entire armature: double tap A

insert keyframe in timeline with: i 

(with cursor on pose window)

-------------------------------------------------------------

11. Hit SPACE to play

copy inserted keyframe, reuse stored mesh coordinates during animation