If you are already familiar with how to setup regular compression-only shocks, the hi/lo shocks aren't that much harder to setup. First - remember that spring preload is used for getting the right sag, or ride-in height setup, and fine-tuning the balance or ski pressure only. Do not use spring preload to prevent bottoming out.
Now that we are past that ... the low speed compression clicker is essentially the same as the compression clicker on a shock that does not have hi/lo clickers. This clicker is used to adjust the compression for low shock speeds. So this is your normal riding down the trail type of terrain, stutter bumps, rollers, etc. Set this for your comfortable riding pace. If you are bottoming out in these conditions, increase the compression. If too harsh in these conditions, soften the compression.
The hi-speed clicker adjusts the compression for high shock speeds. So these would be the conditions for: big jumps, super g-outs, gigantic holes, oh-sh&t moments, running down a moguled out rail grade at high speeds. Adjust this clicker to prevent bottoming out in these conditions. Adjusting this one will have no effect on the low speed, and vice versa.
You can use the stock settings as a baseline and adjust from there. The stock settings are usually pretty close.