I can tell you how V does this, there may be better ways but it seems to work for me. I ask myself is the boundary reasonable? Can I enforce it? Once the boundary is stated then it has to be enforced if breached.
1. I only mention a boundary when it is breached and I know because I feel uncomfortable when something is done. Then it is enforced. 2. Once the boundary is stated and enforced, I change the subject ( called normalising) 3. I use a slow deliberate soft voice (no drama)
Here is a link to some examples that were discussed on ODs thread, they might be useful, there is also a thread on newcomers forum about boundaries too. If you read that thread of OD then OD cleverly enforces his boundaries with his WW.