Depends on what you want to know?
How to mix them? >>> Use your ears and make sure you have decent loudspeakers and a decent room. If you can't hear the lows, don't even try to layer.
How to make them? >>> Read thru the manual of your sampler or DAW. I can't tell you how to layer a acoustical kick drum, e.g. swap or layer the original kick with a second one, when you can't use MIDI because the timing varies.
When I layer I work with (of course) my ears, EQ/filter, envelopes and volumes. It's often just trial and error, because I almost never use the same kick. I also check it regularly together with bass, to prevent phase issues.
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