Everybody will do this differently. This is truly a case of "your mileage WILL vary".
How to not do it
The worst way to do it is to not have a plan. That creates the boiled frog who never jumped out of the kettle because it started out cold and there was never a sudden change in temperature.
The second worst approach is to overly-couple the pull of the trigger with what others do. That makes you vulnerable to the fact that others might be asleep-at-the-switch or to those who are envious of the fact that you though ahead and have the means to reduce contact.
Don't talk too much about your plans. (<sarcasm font>Yeah, here I am announcing it on the internet</sarcasm font>) The crab-bucket phenomena will be just as real as the boiling frog. In this case you will have to fend off a rush of people who either want to join you or, more likely, want you to 'share' your preps before you go dark.
A key factor in when you can roll up the sidewalks involves your level of self-sufficiency. We are retired. We don't have to gird our loins every morning and hi-ho our way to work. We don't have tons of money but our needs are simple.
Rolling up the sidewalks
I decided that we will roll up the sidewalks when a confirmed case is reported in Lansing, Jackson or Battle Creek. More accurately, in Ingham, Eaton, Jackson or Calhoun counties.
The three closest towns/cities of any size are Lansing (population 100k) at 17 miles, Jackson (pop 30k) at 20 miles and Battle Creek (pop 50k) at 30 miles. Those distances are as the crow flies.
Rolling up the sidewalks means a monthly shopping trip and Mrs ERJ and I will have no other voluntary human contact closer than 10' outdoors. The people living in the house excepted.
Parenthetical note: The first shopping trip will involve buying at least nine months supply of dried dog food.
We will continue to take care of mom on Fridays and the occasional weekend.
We would stop attending Mass on Sunday.
Other leaks: Kids
Kubota is taking welding classes at Lansing Community College. The lab work is in a large, well ventilated shop and there is much UV light. The classes are generally 2/3 full and they sit at tables.
Kubota is not working, so that is a non-issue.
Kubota doesn't like crowds and generally hangs out with one or two buddies. Hanging out generally means watching TV.
Belladonna goes to work and is taking 12 credits at Lansing Community College.
School is easy, it is already 75% electronically delivered.
Belladonna's work environment is our greatest exposure risk. She works in a plasma center and has too much contact with donors who already cough, hack, spit and often look like they are desperately in need of their next fix. She wears standard phlebe-tech PPE but that does nothing to protect nose/mouth and eyes paths.
Bella likes to go to a Sports Pubs two Fridays a month with a handful of her friends.
Welding the doors shut
The kids have a choice: They can be in the house when the doors are welded shut or they can be living someplace else.
They have all been informed that Mrs ERJ's and my age puts us at much higher risk of mortality. It will be far more expensive if we get it than if they get it.
Covid-19 isn't Ebola. It is not a civilization-threatening event. To put the current guess of mortality rate into perspective, it compresses two years worth of mortality into a three month time-frame. In any given two-year time span, society can expect between 2% and 3% of the population to shuffle off the mortal plane.
From another perspective, any given company will see 2%-to-3% retire in any given year. Not a big deal.
The big deal is that the attrition will not be planned and orderly. The big deal is that various economic entities will not re-start because they were already dead-men-walking. Unfortunately, many of those dead-men-walking produce critical widgets for society.
Welding the door shut
The trigger for welding the doors shut will be when they shut-down the schools. Both kids go to Lansing Community College. Colleges and universities are likely to try to hobble along longer than the elementary and high schools. We will pull the plug when Lansing or East Lansing schools suspend operations.
Covid-19 has been an incredibly fast-moving disease. So far, everybody has been shooting behind the duck.