Happy to hear you are putting in a permanent slalom course!

If you're going to do the job, it's worth doing right. To do it right, you need, you need an engineer with a transit to shoot each anchor in using a prism on a buoy attached to it. I would not advise putting your course in by GPS. It's just not consistently accurate enough (within 6 inches to a meter!?). The course is going to be in the lake forever, permanent... it will be more cost up front but it's going to last longer.

As far as building a form goes, you can use cement chimney blocks. They are about the size of two cement blocks put together, but only have the outside perimeter set. So they are ready-made forms, plus extra weight. It's a little bit tougher on the budget, but the job will go much faster.

If you're putting in a course that is meant to be permanent, you want wicked heavy anchors... chimney blocks filled with cement will be close to 100lbs. Remember that eye bolts will rust underwater so buy what you can within your budget to make sure they last as long as possible.

Ski rope seems to work great for course applications. Ed Brazil's style is to fid the ski line so that the sub buoy is easily adjustable with a quick tug or slide of the line. Another popular method of changing the subbuoy height is a small piece of pvc pipe, the line runs up through a hole in one end of the pipe, to the sub buoy, and is tied off at the other end of the pipe. This creates a cinch so you can slide the sub buoy up or down. Brazil's fid technique does the same thing with no moving parts to chafe the line over time.

The sub buoys distance from the surface is going to be variable on your ice and boat traffic. In the northeast, about 3 feet of ice is common. Your top end wakeboard boats will draw about the same amount at a dead plow... maybe even 4 feet. So 4-5 is probably safe. If you don't have these issues on your lake and it's just ski boats there, then you can go 3 feet down no problem. *Note that's measuring from when your water level is at its lowest point. If your water level varies significantly you are going to have to adjust the height of the sub buoy more frequently.

If you want to get really techie (if you need to get the course in and out frequently, after every use) you can look into a counterweight and pully system that would sit under the subbuoy. Adding a large ring to your sub buoy makes it easy to hook it with a boat hook. The pully system would allow you to raise the sub-buoy to the surface in order to snap your buoy on for daily use. This takes a little more work obviously, but the convenience factor for removing the course daily (if you have to) is worth it.

Plenty more to this topic that I don't know, post away.