Just done this myself... Not too difficult as long as you don't have fat fingers!
The lock barrel has 8 slots in it (4 each side), 6 are used for locking elements then one empty one then the plate that keeps the barrel in place.
Each element has a associated spring that needs to be inserted first.
There are three different lock elements and there are stamped with a number so if your eyesight is OK you can see the difference.
What I did was work out the 6 required elements and then test insert the barrel without the locking plate.
Once you are happy that it works, replicate the configuration to the other five barrels, add the locking plates and insert them into the panniers.
The barrels can be inserted in two orientations (180 degrees apart) but will only fully seat in one postion.
Just take it slowly and it's an easy job.
One thing I wondered, is it possible to get the barrel out again by moving the locking plate?
Regards,
Richard