harmonizing can be done in many ways.
3rd 4th 5th 6th 7th basicaly whatever u want (but i think 5ths and 3rds are used most, not sure though)
u can play the 3 note from the basic note u play.
---2-3-5-- to harmonize the 2 play a 5 with it. (the 5 is obvious the 3rd note if u start counting from the 2)
just follow the scale and add the 3rd note from the note ur playing ur playing.
So the setting depend on the scale ur playing in
u can use these little chords to get the same effect(settings depend on the scale ur using)
the 8 is the same note as the 3 on the higher string.
harmonizing with a 5th
this ones easy.
say u want to harmonize this
with a 5th note play this
i duno if this makes any sence, im not a teacher so
atleast i did my best to explain