guitars

What helps me is knowing what they are. The strings don't vibrate in a perfect single wave, the harmonics are already there. If you pluck an open string and briefly touch the string at the 12th fret (half a wavelength), you're dampening all the harmonics that DON'T have a node at that position, so the full wavelength and thirds are dampened but the 1/2, 1/4, 1/6, etc are left to ring out.

You just hold the pick in a way that your thumb follows and briefly touches against the string at the 1/2, 1/3, 1/6, etc. nodes. You pluck based on the length of the string from the bridge to the fret you're holding. So if you want to do the 1/3 wavelength, you "cut" the string into thirds and pluck at one of the two nodes such that your thumb "nicks" the string at that node.

A way I used to figure out where those nodes are based on the fret I'm fingering is to lightly rest my thumb on the string, and pluck with another finger (usually ring finger), moving up and down the string until I find one. The node is where your thumb was.