I seem to be the designated driver for the ash mobile over the years. It's real simple, and clean for the inside of the airplane (outside, you're gonna get dirty!) provided you:
1) Slow the airplane down a bit. 90-100 MPH works good.
2) Open your side window, or door, enough to place both hands and the bag, about a foot behind the door post. Do not remove the doors!!!!!!
3) Hold the bottom of the bag firmly (real hard) with one hand, and hold the neck of the bag closed with the other.
4) When you get over your spot, and especially if people are watching from the ground, start a gentle turn to the left (or your side of the airplane) and slowly start letting the ashes release from the bag.
Obviously it takes three hands

so I usually have someone steer for me while I let the ashes go. But from the ground it's really a pretty sight like a long puff of white smoke trailing from the airplane.
I've done it by myself before, but it means really trimmimg well, and being able to keep your hands outside and off the yoke for a minute or so.
And... Don't ask me how I know, but don't ever try and scatter ashes from a helicopter.
Gump