I would think the simplest way to do it is to actually try to reproduce the sound with your mouth and listen to the sounds you create...? ^^; I mean, that's what onomatopoeias are...phonetic transcriptions of sound.
Like, when a character walks, people often use 'tmp', 'clop', 'clak', etc., depending on what shoes they're wearing and what they're walking on. Try to say those in a way that sounds like footsteps...you can hear the connections, right? There's a reason they chose those spellings (and the capitalization...tiny sounds usually get lowercase spellings, or at least small text).
Or for another example: explosions. You could just write 'BOOM'...or you could try to customize it depending on the type of explosion. Make some explosion sounds and write down what you hear, just as an exercise.
'CRRSSSSHHHH' is more like a cascading explosion, like a lot of objects collapsing and falling on each other...'BLOOOSH' would be like an underwater explosion, I think. 'FOOM' would be an airy one, like maybe an explosion of dust, rather than a solid object.
Of course, there are some more abstract sounds I made...'KRRSHKOOW', for one. I could use that, but it's a little complex...onomatopoeias work best when they're as simple and straight-to-the-point as possible. You don't want the reader to spend a lot of time trying to figure out what sound you're trying portray.
The easiest way to simplify, at least for impact sounds, is to start by taking out as many vowels (and other voiced sounds) as possible, try to limit it to the most 'inhuman' sounds. So we take out the 'O' and the 'W', and we have something like 'KRRSSHHKK'. Nice~
You can also simplify by patterning the new sound off a more 'established' onomatopoeia, so the reader will have a more ingrained sense of how to interpret it. So, going off of 'CRRSSSHHH', we can make 'KRRSSHH' (basically, we took off the final 'K' sound, which does make it simpler).
Or to give a better example, if you want to make a new heartbeat sound, and you start with 'boom-boom, boom-boom' (which is a little weird) you can look at the established 'ba-dump, ba-dump' and maybe make 'b-boom, b-boom' or even 'b-bmp, b-bmp' (notice how we gradually shave off vowels and/or add more consonants to refine the sound).
Hope this helps~