One is to let it play out a bit, because often times there's some useful thought in there that if you completely ignore will actually cause more anxiety.

Once you're ready to stop the rumination simply note it. Some of my go to notes for rumination are "anxiety" "social anxiety" "guilt" "doubt" "fear" "obsession" "not useful". (I use "not useful" the most, as it seems to have a non-judgmental quality to it).

So the strategy is to either sit down or stand and note the rumination as it pops up or continue what you're doing but take note when it pops up. Sometimes I do both. It works very well.