The way I do it:
Odd level:
WWWWW
WdWdW
WWDWW
xddWW
WWhWW
WWWWW
Even level:
WWWWW
WWhWW
dddWW
xWDWW
WdWdW
WWWWW
W = Wall
d = dig out
D = dig out. Later channeled out (see below)
x = Up/Down staircase
h = Channel
The channeling will create an up ramp in the middle of the top of the T.
Once this has been dug out, you need to get rid of all the magma safe stone in the top part of the T, or they'll gum up the pump. You can either wait for the dorfs to haul it away to a stockpile (usually takes forever, since they tend to find fancier stone to haul elsehwere), 'd'ump the stone (takes a lot of time as well, as dumping is a low priority task), or get rid of it by using the boulders to create a floor underneath them.
When the boulders have been dealt with, you need to make sure to provide the holes for power distribution between pumps by designating the tile two steps from the channel for channeling on each of the Ts. Note that the channeling will make the top of the T inaccessible, which is why you should wait with this step until the boulders have been taken care of. These channel designations will jump up/down 1 step between each level to hit the correct spot.
The 'x' stairwell is to allow the dorfs access to the sections in a reasonably convenient manner (with extra work you can fit a pump stack, including its access, in a 3*4 aquifer penetration footprint, but it's error prone and tedious).