You can sand the edge until you see two separate pieces, then you can pry the lid off. Alternately, use a side-cutting can opener.
2
Drill a 3/8" hole in the bottom.
This is for the guitar jack. Note: you don't have to put it in the bottom if you don't want. Also note: a conical bit works wonders for this as opposed to an increasing series of drill bits.
3
Solder the piezo to the guitar jack.
Use stranded wire about 2" longer than the height of the can.