Here's how I've done it-

Plug the return (I'm assuming you have an AG pool) and shut the valve for the suction line (or plug the skimmer).

Put a ~10:1 water:acid mix in a bucket. Plug the bottom of the cell (where the cord doesn't enter) with a #11 expandable rubber plug. Put the cell in another bucket and add the acid to the cell so that it covers the plates. See if it 'bubbles up', if it doesn't - you just wasted some acid and time. If it does, wait ~ 5 min. and see if it has stopped. Dump the acid into the cell bucket, rinse the cell and repeat the procedure.

Repeat as needed.

If you don't see any 'junk' on the plates, or if it washes out with a simple hosing - do NOT acid wash the unit!