I'd start by paying having the Penitus Oculatus go undercover in Valenwood and Elsweyr and use the resistances that already exist there to foment a decent sized uprising. I'd quietly assemble my forces along the Northern and Western borders of Dominion-held Valenwood, and assemble my Fleet on around a series of islands due west of High Rock.
Once the revolt is well and truly underway, I drive south with my forces in a blitzkrieg-like manner until the capital of Valenwood is secure. At the same time, my fleet is beginning heavy ballista bombardment of the important governmental buildings in the coastal cities while my specially trained forces are making amphibious landings all over those same coastal buildings. I'd also have a cadre of Mages trained specifically in the schools of Illusion, Alteration, and Destruction ready to go teleport/levitate into the cities. their mission is to make surprise attacks against important Thalmor Officials and Generals so that once they do start to regroup, they are without most of their leadership.
Then with a major blow now made to the Dominion, my armies coalesce around the more major inland cities before they have a chance to fully regroup and hopefully break them before they have a chance to marshal their forces for a counter-push against the sieges.
At least that is how I'd work it if I were Emperor. If I were to lead Morrowind on a course of conquest, the scenario would change quite a bit.