1. Make sure everyone is educated in metric
2. Change the easy things: the paper size the government uses, the units on food labels, the measures legal to use for sales of loose food or other goods, the units the government uses for all types of reporting. (Therefore if businesses want government contracts, they'll need to use metric.)
3. Change other standards, like residential construction, preferred fasteners, wire sizes. Where old measures are required for compatibility, write "24.5mm" in the standard. If the dimension could be changed to 25mm without any side effect, use that.
4. Change other things people see daily: I don't know if doctors use metric in the US, but I assume they communicate to patients in old units. Change the default, but accomodate older people. Change the road signs. Is anything left?
The UK is part way through 4, but has been stuck there for decades.