Find several small issues and blow them up, raising them to the realm wide level. Be certain not to be overly critical, for you dont want to get banned before you get a chance to gather supporters.
Council doesn't share information with realm? Hint at conspiracies. Complain that nobles are being taken for granted.
War not going well? Blame it on incompetent military led by fools who are leading the great realm into a gutter.
Complain about cliques, complain about families hogging positions at the expense of knights who have won glory to the realm... The list is endless. You will surely get some supporters. Befriend younger knights and the older ones without posts, and slowly explain the failures of the government. Once you have a bunch of cronies watching your back, you can aim for bigger fish, maybe a lord or two for added legitimacy to your noble cause. If you did it right, the rest should be history.
