A Fisherman's Knot has the advantage of being really easy to remember
when you've tried it only once. Just what you need.
And it's really easy to describe.
Because it comprises two Overhand Knots working together.
An incredible result from applying what anyone (!) knows how to do -
tie an Overhand Knot...
Try this to get the idea of teaching it
http://www.troop7.org/Knots/Fisherman.htmlTo get neatnest appearance, when you start with two lines with ends
lying side-by-side overlapping, if you start the first overhand knot
going over the adjacent rope, when doing the other end also start by
going over the adjacent. Or under and under. Then the two knots
snuggle together. Anyone will be proud of the ability to do this neat
knot and will remember it well. But that neatness isn't a necessity
to obtain the functionality you require.
Also, "over and over" or "under and under" is the simplest way (?) to
remember how to do the reef knot ("square knot") correctly. You do need
to know a rule with this effect to avoid ending up with a much less
secure knot (typically the unfortunate "Granny Knot").
So the user will remember the rule, sparkling and clear, because they
are using it twice. They'll feel like experts and, well - why
not...?!