I will first recruit Discord, if possible. I will offer him whatever he wants to get him to leave me alone while I complete my plan. If he's unreasonable, I'll turn the Mane 6 against him.
I will then recruit Apple Bloom, an average filly. If she deems any part of my plan to be stupid, flawed, or in any way risky, I will listen to her.
Following that, I will cordially and politely invite Cadance, Shining Armor, Zecora, the Mane 6, and everyone else important to a dinner party in Canterlot.
Then I will simply nuke Canterlot.
Not literally, but there are many equivalents of a nuclear bomb in the Poniverse. I'll drop everything I've got on Canterlot, and bang, bang, bang, yayz. With every alicorn, the captain of the royal guard, and everyone else in any way nice with a hint of power blown to bits, things will go neatly from there.
Following this, I will do my best to destroy the Elements of Harmony so that they cannot destroy me, if new bearers should happen to come along. I will also demolish the Crystal Heart, the Crystal Necklace, and any other highly powerful artifacts of good.
I will thoroughly study the Alicorn Amulet, and if I deem it safe, I will don it personally. If I can recruit Chrysalis, I will do so. Chances are, I will be able to, seeing as I've already conquered almost all of Equestria effectively.
I think my power's been secured from there. I'll also establish my base of operations in the clouds, so any aspiring rebel earth ponies or unicorns will be met with frustration.
Please point out anything and everything I've missed.