Tying a nail knot in the field...
First of all, I always put a large doll needle into my fly box to tie quick nail knots with. Why a needle? You can use the eye to thread the end of the nail knot across and pull the needle out to thread the tag through. Much easier than threading the tag through the loops. Anyway, in my saltwater box after a fulltime guiding year the needle was rusted up and staining my foam flybox so I tossed it. Wouldn't you know I needed it on my most recent charter!
I started to tie an albright but then realized the end of the flyline doubled over forms a very nice channel, like the gap between two fingers when held together.
After wrapping 6-8 wraps of butt, I simply slid the end of the nail knot through the gap between the two folded halves of the end of the fly line. The nail knot was ready to go, but no need for the double line so I then pulled the loose end out to end up single again through the nail knot ready to be tightened. Pull the fly line closer to the awaiting knot and voila! Pull both ends tight (opposite tightens opposite), trim tags and you're fishing.
It's really very easy to do and I don't even think I'll bother with a needle anymore!
Works for me.