Very simply put, proper blowing involves the constant, appropriate supply of air to all the reeds. This incorporates exhaling (blowing) into the bag and maintaining a constant bag pressure during and between breaths.  This constant bag pressure is maintained by squeezing the bag gently with the arm while inhaling and decreasing the arm pressure while exhaling into the bag. This takes considerable practice and perseverance. Your ability to maintain steady bag pressure can best be measured by listening to your drones. They will react to the slightest changes in air supply by falling in and out of tune. With practice and good instruction, you will master the technique and steady blowing will become second nature to you.

Inexperienced pipers should master steady blowing by playing the slowest of tunes that they know and listening to their drones. In order to better focus their attention, they may wish to turn off one or two drones and listen intently to the remaining drone or drones. The piper should be comfortable with the feel of the bagpipe. There should be no conspicuous heaving, rocking, swaying, or bellowing of the bag arm. A great deal of practice, with the emphasis on steady blowing and not on playing tunes, is necessary to master blowing. The key.....listen.
This sound file (above) was recorded in Nobel Victory Memorial Chapel at SJNMA using one of the first Sony mini-disc recorders.  I held onto the long notes excessively long to demonstrate steady blowing.  I played my silver & ivory Bowen Classic with a MacLellan chanter.  The 4/4 is a tune I wrote called "Olivia" named for step-brother Mike Szarka's daughter.
My Bowen Classic by Dunbar. The silver was done by Dave Davidsee of True Hand Silverworks.  The pattern was inspired by silver on an old MacDougall bagpipe.
NRPPB in a memorable performance in a downpour in Kincardine, 2000. I was playing my full silver Maclellan on the back corner. I think everybody took a step away from me when the lightning started hammering away. We played, laughed, and then drank beer. Life was good.
Blowing Tone