I would start by taking Dragonstone. It would be a major propaganda victory to take back the Targaryen ancestral seat and a large number of Tyrell forces would be obliterated by the dragons (assuming they are full-grown). I would not put any Unsullied on the ground until the Tyrells holding the castle surrender and open the gates, in order to preserve the force. I would then do what Aegon I did and send out ravens that Westeros will be ruled by One True Queen (I dont support her claim, just hypothetically saying what I would do) and that the great houses can bend the knee or be destroyed. Assuming the Martell's bend the knee (im going by the show here so Faegon doesnt exist at the moment), I would have them start harassing the reach with raids. Due to lack of manpower, I would use my dragons to demoralize any opposition in the Crownlands, the Westerlands, and Riverlands, eventually out of fear or greed, most of them will join my side. Then I would offer Sweetrobin a ride on one of the dragons in return for the Eyrie being surrendered peacefully. The Reach would then be basically surrounded by enemies, with Dorne to the south and the Targ loyalists all around it, and the Tyrells would eventually surrender or be destroyed. Then I would obliterate the Freys with my Dragons, because 1.Fuck the Freys 2.Ill be going on the kingsroad anyways. The North would be a very sticky situation, but I think Roose Bolton is a calculating coward (with apologies to r/dreadfort) and wouldnt risk his neck to fight me, and he would bend the knee. If there are certain houses with a history of betrayal (such as the Boltons) I could remove them and put in my friends one by one once the main war is won. For example, once the continent is conquered, I could rally Northern houses and help them take back Winterfell at minimal cost to me or my forces a few months later. I will not go into the White Walker threat in this post because we are talking politically I assume.