Dig up the mums with a garden shovel. Each mum's root ball will be about the same diameter as, or slightly smaller than, its foliage.
Lower a mum into an empty pot. Estimate how much soil is needed at the bottom of the pot and set the mum aside. Fill the pot with the appropriate amount of soil from your garden.
Set the mum in the pot. Add or remove soil from the bottom of the pot as necessary to allow the mum's base to rest just below the pot's lip. Add soil to the gap between the mum's root ball and the pot.
Press the soil down slightly with your hands to compact it in the pot. Add more soil, if necessary. Water the mum until water runs from the pot's drip holes. Water the mums every seven to 10 days to keep the soil moist.