First:

Be normal Spyro

Second:

Choose whichever element you want (Excluding Convexity)

Third:

Press Square or Triangle

Fourth:

Press L3 (It's the left analog stick)

Fifth and Finally:

Watch Dark Spyro use the same element moves but more powerful and a lot cooler looking

Notes:

• I have not been able to get all of the attacks to work (Maybe I have and I'm just a retard and didn't notice).
• Just in case anyone didn't know (and I'm not trying to be rude someone actually asked me) what the Square and Triangle buttons do I'm going to tell you: Square does the breath attack and Triangle does the other move thing that I don't know what to call.
• Again just in case Convexity is Dark Spyro's element.
• You can get Dark Spyro by completing the story mode thing and then beating all five dragon challenges (The five dragon challenges are pretty easy I just used the electric breath bomb orb whatever it is except on the first when you randomly switch elements).
• Pay attention to the attacks you may have done it and just didn't notice.
• And the ice secret attack, when you use the polar bomb move away from it you will know why when and if you try it.
• L3 turns you into Dark Spyro during gameplay after you unlock it and load your save file that you used.