Hi, welcome to the forum. To flush, you run three times as much water as your container size, through the pot. Example - if you have a one gallon container, run three gallons of water through.
If you have a drill drill in about three holes they don't have to be really huge. There is not set number on it though as long as you get the excess water out. Make sure you have bowls/trays to catch the water underneath or you will have a big mess!