Set the width of the clamp, so it's the same as, or just slightly wider than the knife. Tighten the top screw first, then the bottom screw, to lock the blade in.
You also need to set the knife so that the amount of metal removed as you approach the tip is consistent with the rest of the bevel. To do this, the knife is adjusted horizontally, (changing the distance from the clamp to the tip). This can be done by marking the edge with a Sharpie (see "Setting the Angle" section), and adjusting the knife so the marker is removed along the entire bevel, or center of the bevel, as you approach the tip. A description of this method can be found here: How to find your angle.
Once you have the knife properly positioned, you'll want to record your settings for future reference. This is one of the key concepts of the Wicked Edge... the ability to quickly replicate your previous work, so you can quickly touch up a knife without having to grind in a new edge every time.
Additional Info on Clamping the Knife
The following tips (in bold) are from an excellent post from forum member tcmeyer:
I've watched a few videos of WEPS users at work on this site and believe that most don't really understand the principles of the vise mechanism.
The designer intended that the top screw (the clamp screw)is used to set the clamp distance (the opening between the jaws), and the lower screw (the jack screw) is used to create the very high clamp force required.
To avoid excessive wear and galling of the threads, the clamp screw should be used only to apply enough force to position the blade. The jack screw is used to create very high clamping force by means of the lever principle. Think of this as a common pliers. The clamp screw is the pin (fulcrum). You apply an amplified force to the workpiece by applying force at the far end of the handles.
Keeping this in mind, there is no need to apply more force than necessary to the clamp screw. Except for very large blades, it needn't be tightened more than snug - just so the blade stays put.
The jack screw is capable of producing oodles of clamp force with only a modest amount of torque. For most small-to-medium blades only about a quarter of a turn past the point of contact is required. I believe this is also what is recommended by WE. If you apply too much torque, you are applying excess bending stress to the opposite jaw. Remember that every extra 1/4- turn bends the clamping jaw that much farther.
The threads in the aluminum alloy clamp aren't intended to endure the very high frictional (galling) forces created when high amounts of torque are applied to the screws. Moving a hard metal across the surface of a softer metal at high pressure results in galling, where bits of the softer metal are torn from their natural positions. This is often what happens when you "strip" the threads of an aluminum block. Extend the life of your vise by using only the required amount of screw torque.
I suggest also that you lubricate the screw threads with some anti-seize grease. I use RCBS re-sizing lube.