To join a ruler, you need to do the following:
First, you need to role play with them in their audience hall or any other role play thread. The final RP (role play) that you need to reference has to either contain or be after both you and the ruler have agreed on terms for joining him or her. Once you have identified that post, you need to retrieve the post ID to reference it when you join the ruler. To do so:
Once the button is clicked, you should see the post ID as such:
Armed with this information, make your way to the OM tool (OM link can be found at the top of the site with other links under the Tools heading: Tools: KT • OM • TC • IS • MP • MT • DT • CG • PT • MA • BT). In the OM tool, select the ruler you just role played with and enter the ID in the textbox presented and click Join
Once that completed, the ruler will simply from his KT tool accept or refuse you. If accepted, you will be added to his force, if not you remain a free officer. His response will be posted either way in this thread for reference including the role play post.
As a final note, officer can also join another ruler or become a free officer in the same way. If free officer is selected, he becomes a free officer immediately, without need for confirmation or anything.