Just Make Sure...
you remove the "plug" to the vent line after you install the new / repaired drain. Bad things happen if you forget it.
As mentioned above, if you have a spare drain ready to go, using a finger to cap the tank while maneuvering the replacement into position then quickly removing the finger and getting the drain started results in minimal fuel loss.
I've had to replace three O'Rings so far using this method.
If you get a spare, I recommend "opening" the valve, insert a nail, pop rivet, etc. through the hole; this relieves pressure on the O'Ring while sitting in your kit bag. I do this because I don't want the O'Ring taking a set while dry in storage.
I'm sure you'll get lots of recommendations, so do what feels right / best for you.