https://www.amazon.co.jp/kindle-dbs/ku/ku-central/ref=ku_cip_aw_kc_rtn and click the button under your credit card that say "Kindle Unlimited <jap characters>" which leads to confirmation page. I don't really understand it too well but I think if you end it early, you cannot download anymore unlimited manga but are still able to keep what you downloaded until membership officially ends in 30 days (in which the download manga get deleted).
What are you confused on? If it's making a JP acc then idr what I did but you can either use chrome fore google translate or there's an option to change the language on amazon. If it's getting KU then find the yaoi on amazon and it should have an option like 0¥ and click on it, should prompt you for CC, then done :P
If you guys are desperate for raws like I was, you can make an amazon JP account and do the kindle unlimited 30-day free trial so that you can access it (+more yaoi hehe). Just make sure to cancel the membership before the trial ends unless you don't mind paying!