For simple free-floating ones I do them like this.https://www.youtube.com/watch?v=SdG3T0I6Pd8
The only thing I do differently is put some reflective tape on there so they are easier to find since they have a tendency to swim away.
For the flagging ones, the jugs that pop-up when bit, I use 9" of backer rod and drill/force a hole down the middle with a knife honing steel. It just bores a hole down the middle of the backer rod. I go about 1/2 way then do the same from the other side. I will stick my middle finger in there from both sides to stretch it out a bit after. Then you can put the pvc down thru there pretty easily.