When I've done this (not often) I've started at one end of the
board, applied hot glue around the first six inches or so of holes,
laid the leather on and dished it using a block of wood turned with
a dome on it and a rim to ensure sealing all the way around, then
paint the next several inches with glue and proceed.  I've usually had
a few wrinkles between pouches but I really don't think they matter,
except perhaps aesthetically, as long as each pouch is completely
sealed around the edge.

I'm sure this is a technique that would benefit from a lot of
practice.  You need to be quick and have to get the leather down and
the pouches dished in the minute or so before the glue hardens.  At the
intersection of pieces of leather you can glue on on top of the other
then cut through both (in the space between pouches) with a razor blade
or sharp knife, pull the tag ends out from underneath at re-seal the
top piece for a near-perfect looking joint.

The biggest problem I've had is using too much glue, having it run
down the side of the pouch well and stick the leather to the side
around the rim, limiting pouch travel and possibly creating a wear
issue for the future. Since the pouch only has to move the valve 30-50
thousandths of an inch this hasn't caused a valve not to work.

Cheers,
Roger Wiegand
Wayland, Massachusetts, USA
http://www.carouselorgan.com/