I have been wanting to make one of these for a while, but never had the motivation until today, when I decided that I had nothing better to do. It is basically a fence with a 30º outward pitch in order to prevent snakes from entering your garden/yard/whatever. I only built one panel for this but it will get you started or at least give you ideas.

DISCLAIMER: As with anything, be safe and smart when working with tools. I am simply showing my design and it is by no means the perfect setup. More support may be required to keep yours up, especially if different materials are used. I also cannot guarantee the success of this fence, since many factors would be involved in your specific situation. I simply built mine for the many non-poisonous snakes that hang around. If you have poisonous snakes near your garden, your best bet would be to have a professional remove them,

Step 1: Source Supplies

I used some scraps I had lying around (hence the random hole in one sheet of plywood you may see in pics). For mine I used:
Two sheets of 20"x25" plywood
One 2"x4"x12'
Some screws, any will work, I used some self tapping, 1-1/4" outdoor screws, as well as a 3" deck screw, but that isn't necessary.
An old piece of scrap wood to reinforce with, I used an old fence board.

Tools needed:
Drill
Saw, anything electrical is best. I also used a Miter Saw for angle cuts.
Tape Measure
Pencil
Square, really not necessary, I just used it to check angles

Oh yeah, and always have a first aid kit handy. You never know...

Step 2: Prepare the Main Panel

For mine, I laid out the plywood and measured 6 inches from one end on each. This is going to be the part buried underground. Since my plywood gave me an overall width of 40 inches, I cut a 2x4 that length and attached it using 4 screws just above the 6 inch line. Attach another at the top (NOTE: you could do without this if you don't want it top-heavy, but the design would change. I did it this way for a reason I will get to later).

Step 3: Attach Support

Cut a 2 foot section of 2x4, with one end at 30º angle. Find the middle of the panel and attach this 2x4 with the angled end. I pre-drilled some holes to make it easier. Next, cut a 30.5" 2x4 and attach to the opposite end of the 2 footer, vertically, with the horizontal 2 footer butting up against the side of the vertical, 30.5 incher. This is where I used my 3 inch screw, just for some good support. You can reinforce this however you wish, but I used an old fence board, cutting two pieces with 45º angles, and screwed them into the sides. You should now be done with this panel. Make more if You are covering a larger area. I have too large of a garden for this to be practical, but might cover a section along my ditch.

Step 4: Bury It

Make sure you dig down 6 inches, digging a 'T'. Put the panel into the hold and start to fill the hole back in with dirt, packing it down really well. You will notice that when buried, it doesn't seem so top heavy. The top upside-down "L" shaped support would make a great base for a garden table, for potted plants or decor, just make sure not to put too much weight towards the outer edge if you do this, as it will probably fall.

One final thought: Keep the foliage down around the outer edge of this fence, or else snakes will simply climb over it and make it useless.