Harmonics can be kind of amorphous not just in sound but in interpretation. There's a lot of variance in how composers notate them. And it can be complicated by the fact that the bass is actually a transposing instrument, sounding an octave lower than written.
But generally, there's two basic flavors of natural harmonics.
1.) An "o" over a regularly written note just means the actual sounding pitch should be at that note, but it's up to you to find a harmonic that gets you there. Often there's more than one solution.
In the first example you'll get that "D" just touching the octave position on the D-string. The same with the 8th note "G" in the second example: touch the octave on the G-string. And last, the same for the "E" on the bottom line of the third example: touch the octave on the E-string.
2.) The second system uses a diamond to show where to place your finger and other indications for which string to use and what the actual sounding note is;
So the second example, touch the indicated "C#" on the A-string (it sounds two octaves higher than the fingered position), and the indicated "B" on the G-string. Also which you didn't quote, the same for the "F#" diamond that follows 5 measures later: touch the indicated "F#" on the D-string.
Before hitting the third example, what I just told you to do...don't do it!
Those particular harmonics sound much clearer and are more reliably found and bowed at the end of the fingerboard close to the bridge. Remember the more than one solution? They're written where they are for visual clarity, but most players will finger them at the bridge. The advantage not just sound quality but also the harmonic notes line up exactly with the notes you would get pressing down to the fingerboard there.
If you touch each of the strings with your thumb at the two octave position, you'll sound the harmonic that actually corresponds in pitch to that position on the fingerboard. Just move your thumb across the strings, hitting those two octave harmonics to get used to establishing that position. And I cheat: I mark the position with a penciled in "fret". Now without shifting, use your first finger to touch the string a major third higher than your thumb (but lift your thumb, only one finger at a time touches for natural harmonics). Again, the harmonic you sound will correspond exactly to the note you get if you press down there: two octaves and a third above the open string. Lastly, again without shifting, touch the string a fifth above where your thumb is. Same old, same old: the harmonic will correspond to the fingered note at that position (two octaves and a fifth above the open string), though most likely you'll be off the end of the fingerboard. I use my second finger because I'm missing the pad on my third finger, but you might be more comfortable using your third finger for that fifth. Whatever gives you the more relaxed hand shape.
So just spend some time to get use to finding that two octave position and fingering the root, third and fifth on each string with your thumb, first and second (or third) fingers. Experiment with bow placement, weight and speed to get the clearest sounds.
So, the second example redone, the harmonics are fingered with the first finger two octaves and a third on the A-string, G-string and D-string respectively. Easy. And you also might want to try them as notated to see the difference in ease and sound.
Last example: bottom line, again, touch the octave on the E-string, the sound is the indicated "E" printed there. Middle line, touch the diamond "B" on the E-string, the sound is an octave and a fifth over the open string, i.e the "B" indicated just above the staff. Top line, notice that's one of your tuning harmonics. You can either touch the diamond "A" on the E-string and it will sound the indicated "E" two octaves higher or (again, your tuning harmonics) touch the matching "E" or fifth on the A-string for the same note. It might be clearer there. I'll leave it to others to argue which harmonic fingering gives the more authentic sound the composer was thinking of.
BTW, wait until you get to Ravel. I've actually read someone's doctoral thesis trying to sort out his harmonics!
