Put $deck_numPieces$ in a text label on another map?

What’s the easiest way to accomplish this? I see from scouring forums that I need to likely use a triggered key command to update a global property, but I’m very very new to Vassal and don’t really grok the way the code works.

Specifically, I’m working on some improvements to the Babylonia module. I’m going to add a draggable player-color “play area” to the main map window, and (among things I’m going to do with it) I want that player area “piece” to have a text label in a corner somewhere that reports the tile count in the player’s deck of pieces within their own player window.

It’s not overly complicated. There’s just a deck in each player window with a counter that simply decrements each time a tile is drawn at the end of the turn; I just need to pass the $numPieces$ value/object between the windows.

But I need the for-idiots version of how to make this work.