To enable Y!social linking - and by extension crossposting/syncing, head over to your account settings.
There, you’ll see some new options for Y!social linking and options to control what gets posted and if syncing is enabled.

Then, you can choose if you want just journals to cross-post automatically, or if you want both submissions (excluding extras) and journals to cross-post.
Additionally, you can enable syncing. This is where the Y!social post will be kept up-to-date with the original post/journal - filters and all. It will even delete the post on Y!s when/if you delete the submission/journal

Finally, if you choose to keep sync enabled but don’t want auto-posting, you can disable auto cross-posting and check the box beneath the journal or submission to post it for you that one time.