Depends what you want to achieve.
If you want to limit compression (up travel) to stop the chassis hitting the ground, ideally take the bottom fixing off the shock shaft and slide o-rings onto the shaft.
If you want to limit droop (down travel) eg, to stop driveshafts falling out, then you'll have to disassemble the shock shaft from the shock body and slide o-rings onto the shaft before popping it all back into the shock body and reassembling.
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