The arbor knot is the knot you use to tie line and backing onto your reel. It’s very simple. (Click on the photos to enlarge them.)
Pass the line around the arbor.
Tie an overhand knot in the standing line.
Tie another overhand knot in the tag end.
Moisten the knots (saliva will do- the lubrication keeps the line from weakening from the friction of tying.) Hold the tag end on either side of the knots and pull tight. Cut off any excess line above the knot in the tag end. Snug the knot down. All done.