I first punch out the nib as described by several others, but then I just slide a spare 7mm tube over the tranny all the way to the tube in the blank. Since the tubes are identical diameter, they meet up perfectly.
Then I stand the assembly up vertically with the spare tube resting on a hard metal surface - like an anvil, table saw table, etc. Then using an a punch that just fits into the tube, give the punch a few light taps with a small hammer & the tranny will slide right out into the spare tube.
If I'm completely removing a failed transmission, I just drive it into the spare tube & discard both. When I just need to back it out a small amount because I pressed it in too far, it only takes very light taps to move the tranny a mm or so. Generally it will be possible to easily pull the spare tube off the tranny when it's only been pushed back that small amount.
No special jigs, simple tools and really quick. I keep plenty of spare transmissions & tubes on hand. If you try this method, don't tap very hard at all if you just want to move the tranny a small amount.