In her 1976 book Meditation Without Frills: A Woman's Workbook, Maureen Jones-Ryan describes a mantra like this:
"A mantra is a gentle, soothing sound on which you concentrate while entering into the meditative state. This meaningless sound is simply a tool used to prevent the processing of thoughts".
In that same book, Jones-Ryan also includes a format for creating your own personal mantra. If you want to learn or see that you'll have to track down this obscure publication. However, this inspired me to devise my own formula!
Formula for Creating a One-Syllable Mantra
Select one of these seven different sounds for the beginning of your syllable: h, l, r, s, sh, y, zh (note: "zh" is like the sound in the word mirage).
Then select one of these nine different vowel sounds (spelled phonetically) for the middle portion of your mantra: ah, ay, ee, eh, ih, oh, oo, uh, ul. (note: "ul" is meant to be like the sound in the word pull). Some say that ah, ee and oo are the most pure vowel sounds. Hint, hint!
And finally, select one of these three lingering end sounds for your syllable to resolve: m, n, ng.
So, for example, you might choose y + ee + ng to form the mantra yeeng. Spell it however you want in your mind...ying. Or don't spell it at all. There's your answer! Oops, don't give it any meaning. It's just a sound. Some combinations might remind you of an existing English word. Like hohm or rayn. If that connotation has a good feel to you, then fine, but to get below the surface it may be better to find a combo that has no perceived meaning in its sound.
Elsewhere in her Meditation Without Frills book, Maureen Jones-Ryan says, "Do not repeat your mantra in rhythm with your breathing. They should not be consciously connected." If you struggle with separating your mantra from the rhythm of your breathing, then a one-syllable mantra may help. The longer the mantra, the more effort people tend to add.
However, some may prefer a two-syllable word sound as their mantra. So Hum is a well known two-syllable mantra, so this is certainly not without precedent.
Formula for Creating a Two-Syllable Mantra
Begin once again with h, l, r, s, sh, y, or zh. Then just like before choose ah, ee, eh, ih, oh, oo, uh, or ul. (This becomes your first syllable). Start your 2nd syllable with the same choice as the first: h, l, r, s, sh, y, or zh (this step may be optional in your case). Then choose another vowel sound, either ah, ee, eh, ih, oh, oo, uh or ul. Lastly, end with m, n, or ng.
Example: sh + ih + r + ee + m equals shireem.
You can have a lot of fun with this! Try several combinations until you find the one that appeals to you the most. Then stick with it! The mantra itself has no power. You give it the power!