For any harmonic, you first need to get a sound source of that frequency such as a tuning fork or speaker. It’s best to place the source where you expect an antinode to be. You can try to just pluck the string at that point but that will probably also produce a lots of harmonics you don’t want.

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For the kth harmonic, there are k antinodes at (2i-1)/(2k) of the string length, where i≤k; i∈ℕ.

Fundamental (A1):

• ½ the string length (fret 12)

2nd harmonic (1 octave up, A2):

• ¾ the string length (fret 4.98)
• ¼ the string length

3rd harmonic (perfect fifth from A2 or approx. E3):

• ⅚ the string length (fret 3.15)
• ½ the string length (fret 12)
• ⅙ the string length

4th harmonic (2 octaves up, A3):

• ⅞ the string length (fret 2.32)
• ⅝ the string length (fret 8.14)
• ⅜ the string length
• ⅛ the string length

8th harmonic (4 octaves up, A4):

• 15/16 the string length (fret 1.12)
• 13/16 the string length (fret 3.59)
• 11/16 the string length (fret 6.49)
• 9/16 the string length (fret 9.96)
• 7/16 the string length
• 5/16 the string length
• 3/16 the string length
• 1/16 the string length

Using fractional frets is cumbersome because they are non-linear. You’re probably better off with a tape measure or ruler.