I just bought one of these on ebay for $10

http://cgi.ebay.com/Timeless-TR-900-Prong-Arrow-Rest-Right-Hand_W0QQitemZ7118430898QQcategoryZ36099QQssPageNa meZWDVWQQrdZ1QQcmdZViewItem

I have an old bear whitetail II...the screw from the rest fits perfectly in the bow...my question is how do I tune this thing?...it comes with 3 tiny allen wrenches that can make adjustments on different parts of the rest...should I just start messing with it in conjunction with my 3 pin sight until I start shooting good groups or is there a better way to go about seting up and tuning a dual prong rest? Should the odd color fletch point down or out? Thanks for any and all help

OK....it's difficult to explain, but not difficult to do. This rest is very basic so the adjustment isn't quite as finite and simple as on some others. First, the cock vane goes down, and if you have much helix on your vanes......you're gonna hit the prongs. You gotta fletch 'em straight or a slight offset, no helical.

Now, to tune this rascal. The basics are the same.......left, right, up and down. Head to the shop to do some paper tuning. Depending on the tear, loosen the small allen screw on the horizontal post moving the rest either left or right. Be careful, now....because this also moves up and down. Same screw. Ideally, you don't want the prongs standing straight up. Another way to adjust up and down is to loosen the mounting bolt on the riser and move the whole thing. It's not all that tough, but since it's so basic it can get aggravating. Just really take your time and you can get it. I shot those style rests for years. It can be done, just be patient.

Remember, when you loosen the allen screw to move it left or right, it will also rotate up and down, so hold on to it and try not to let it move up or down. Hope this helps. Good luck!

Sounds like mobo explained it pretty well.
I would add though that the arrow wants to sit on the prongs,not inside them if I am making this clear,hopefully.The prongs are adjustable to open up and close to accomodate different diameter shafts,have it so that maybe an 1/8 of the shafts diameter is in the prong.
You also want to have the arrow on the prong and try to level it with the berger holes,thats the mounting holes where the screws connect the rest.
Another important aspect is to have it centered of the riser and string.This is called centershot.If you dont have the proper tools for this job and want to get it as close as possible then nock an arrow,have it sitting level with the mounting holes,hold the bow as though you are going to shoot it.Now,line the arrow up with the string and adjust the rest to center as close as possible.This will get you in the ball park and make tuning and sighting in much easier.
Better yet,take your bow to a shop and for real cheap have the pro do it,explaining what he is doing along the way,get involved with the setup as much as possible and you will be shooting good groups in no time.
Oh--the odd colored vane(fletching) is the cock vane,it will point down.

In regard to centershot placement, some bows have a slight offset to centershot. That is, due to the string offset on some cams, the centershot may not be true center in relation to the limbs. Check your bow manufacturers website they may have info ( or just "Google" "centershot adjustment Hoyt (or whatever) bows".