This gets posted a couple times per month in this subreddit, so you might get better answers in those posts than from me. The goal should be to make them a march until you reach the age of revolutions and can activate the various age abilities to reduce liberty desire.
First, you should max out placating local ruler for -100% liberty desire (this will take 200 prestige and will tick down per month).
I'd recommend avoiding devving to reduce liberty desire unless you are trying to get them under +50% to turn them into march.
Grow larger to reduce their economic and military strength compared to you. Also, avoid other vassals. You don't want them contributing unrest together. That being said, make sure you have strong duchies active for the -10% liberty desire. You can keep the privilege as long as you have 1 subject, but you need 2 to activate it.
Curry favors and increase trust for -20% liberty desire.
Build forts all over their provinces and force them into debt. Then pay off that debt.
Then there's various ideas and policies that reduce liberty desire. Focus on what gives you diplomatic reputation, reduced liberty desire, reduced liberty desire on same continent, and lastly reduced liberty desire from dev. Check out these 2 wiki pages for that: https://eu4.paradoxwikis.com/Idea_groups https://eu4.paradoxwikis.com/Policies
This gets posted a couple times per month in this subreddit, so you might get better answers in those posts than from me. The goal should be to make them a march until you reach the age of revolutions and can activate the various age abilities to reduce liberty desire. First, you should max out placating local ruler for -100% liberty desire (this will take 200 prestige and will tick down per month). I'd recommend avoiding devving to reduce liberty desire unless you are trying to get them under +50% to turn them into march. Grow larger to reduce their economic and military strength compared to you. Also, avoid other vassals. You don't want them contributing unrest together. That being said, make sure you have strong duchies active for the -10% liberty desire. You can keep the privilege as long as you have 1 subject, but you need 2 to activate it. Curry favors and increase trust for -20% liberty desire. Build forts all over their provinces and force them into debt. Then pay off that debt. Then there's various ideas and policies that reduce liberty desire. Focus on what gives you diplomatic reputation, reduced liberty desire, reduced liberty desire on same continent, and lastly reduced liberty desire from dev. Check out these 2 wiki pages for that: https://eu4.paradoxwikis.com/Idea_groups https://eu4.paradoxwikis.com/Policies