Hi there, first post for me on the forums!
Been playing lots of Daimo runs lately, and I mean lots. However the only Daimos I have been able to conquer Japan with are Hosokawa and Ouchi. Since I'm probebly not that great of a player I guess my way of doing it is wrong but here it goes. Starting year 1444:
With Hosokawa you'll be running solo most of the time since pretty much everyone hates you. Start with allying Date and Takeda, pause the game when the first diplomat arrives home and go to war with Yamana before they get any allies. Exterminate their army and carpetsiege all their provinces and take everything except their capital. Take all their gold and tradepower.
Start forging a claim on Ouchi and wait for them to join some stupid war somewhere. If you're lucky they attack you and you will have some allies. Otherwise you'll probebly loose Date as an ally and they will join them. Not really a problem since Date sucks. This can go either way and sometimes you may lose the war and probebly a province. If this happens I'd restart and repeat the same steps again since you'll get steamrolled later on (in my experience). If you win the war, annex Ouchi. After that, just forge claims on everything around you whilst bulking up on dukats. War the ones who attack your allies or you and slowly take controll of the western Japan.
Get nine provinces and stick with those, Japan will only attack you if you have ten or more provinces from what I know. And now starts the waiting, start wars on other Daimos who are getting more power and force them to return provinces they've conquered, taking all their dukats is nice too. Now you should get like 600+ dukats and then wait for some rebels or something so the Japanese army splits up and then hire mercenaries. Just tons and tons of mercs and go to war on Japan. Quickly kill off any group of armies whilst blocking off all ports for warscore. Siege every japanese province and annex them.
Ouchi is another story, you'll have more allies and this should be abused. Go to war with Hosokawa (reconquest) and only take one province. Hopefully your allies will win this war for you, if not you're screwed. Get some claims on Yamana and pretty much take one province from them for each war, with the help of your allies of course. After that everything is situationall but get nine provinces and rince repeat the Hosokawa tactics.
Some random events will completely screw you, like leaders dying on you too soon. I can't say that it's easy to do this, but hey, there are worse countries to play our there! I hope this somewhat helps atleast!
/Fullbordad