I've built one before and they're certainly fun, but tend to not be very practical considering the intense amount of gear, time and work it takes to set it up. That being said, here is how it is done:
1) Somebody has to swim one end of the rope across, this is usually a skilled person and a knowledgable person with anchors. The line should also be static to reduce strecth when people are cross. The person swims and end across and ties it to a fixed point. This can be placed gear or natural protection. I usually have doen a no-knot knot (a couple of wraps) around a tree and it works fine.
2)In order to tension the line, you need knowledge of mechanical advantage pully systems. I have set up a 2-to-1 (C pulley) or 3-to-1 (Z pulley). If you have an ATC with guide mode, this can be used as a progress catch for tensioning the line (in replacement of another progress catching prussik). I can attempt to explain how to set up either of these systems, but it will likely be easier to see it diagrammed out online. I've attached a diagram, and if you need it I can give you additional info in setting it up.
Note: A 3-to-1 system means that for every 3 feet of slack you pull out to tension the line, the line will tension for one foot and the 2-to-1 means the same thing but 2 feet instead. As a result, it will be easier to pull on a 3-to-1 but will require more pulling. These emchanical advantage systems are the bread and butter of tensioning the line.

3) Looking at the "pull here" line in the diagram. Do exactly that, pull there, and it will tension the line. Remember that each time your prussik reaches the end you will have to stop and adjust the prussik. The one all the way at the fixed point is your progress catch, you must tend this to save the slack you have pulled out. Tending the other purssik simply gives you more rope to pull out. ATC guide mode again, works excellent as a progress catch in which you do not have to tend. I have used it quite well in a vertical climbing and rescue environment, and considering the mechanics and where the forces are I believe it would work well in this horizontal apllication as well, but haven't tried it myself.
4) After tensioning the line, the last step is essentially to tie off the ends and secure all the prussiks. Simply attach a hanress via the belay loop on a pulley (or carabiner if you don't have one, but there will be more friction) to the line. And slide, slide, slide yourself across, there will still be stretch no matter how well you tension it, and it often feels as though you are constantly pulling yourself up a hill. Some strength as a v2/v3 if you do it right, more like a v6 if you do it wrong. It is nice to have a progress catchign pussik or ratcheting pully, ascendor tibloc etc. in order to save your progress when crossing.
Hope that helped a little bit, and let me know if you have anymore questions.
Cheers,
-Jake