Mudder,
It's really very simple, assuming the button or whatever you're going to use is calibrated (a fixed diameter) and matches the diameter of a Forstner bit you've got laying around...
After the closed-end blank has been drilled and the brass tube glued in, make sure the blank is square and the tube hole is dead center. Then re-mount the un-turned blank in your pen drilling vice and drill a shallow hole in the end of the blank (the closed end side of the blank) with an appropriately sized Forstner bit. At this point, make sure the button will fit in the hole. Then mount the piece on your lathe/pen-chuck and use the dimple at the bottom of the hole (counterbore) you've drilled to align you're tailstock live-center point.
If you've got an odd sized button, you can still use this process, with a Forstner bit that'll leave a hole that's close but not quite big enough. Then you'll have to widen the hole for the button to fit, on the lathe. The Dancing men button is about 11/16", so I pre-drilled a 5/8" counterbore, turned the pen, then did some very careful widening of the 5/8" hole using a skew as I finished the blank.
Hope that helps...