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1. Apply mirror modifier in object mode

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

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2. Add armature

Click on the joint ball - extrude with E

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3. Select bones and Duplicate within menu

Mirror duplicated section

Align by pressing x, y, z + snap

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4. Display bone names:

armature data -> viewport display

[x] Names

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5. Connect a duplicated bone to the original joint

Bone properties - Relations:  [x] Connected

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6. Parent mesh to armature

select mesh + armature (Ctrl)

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

alternative 1: With Empty Groups

alternative 2: Automatic Weights

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

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7. Assign vertex to bone (With Empty Groups chosen)

Select mesh in object mode

open the Data section

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

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8. In object mode -> select Armature

enter Pose Mode

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9. Open Dope Sheet in another window

change mode to: Action Editor -> click new

(several animations for one object)

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10. Rotate bones in Pose Mode to animate

select entire armature: double tap A (old blender version)

insert keyframe in timeline with: i 

(with cursor on pose window)

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11. Hit SPACE to play

copy inserted keyframe, reuse stored mesh coordinates during animation