The star has markings with dots. No dot 1 dot 2 dots and 3 dots. Which do I use to mount my M203?

The star has a groove around it. Does that face forward toward the flash hider or back toward the receiver?

What are the curved washers for?

Read the instructions.

The spacer has multiple thicknesses to allow for proper mounting of the M203.

It didn't come w instructions.  I put it on wo the washers and it looks good.  But when I put The square piece in the space w the grove facing forward it doesn't seem to get back enough to fit in the m4 barrel cut out.  If I remove the square piece it fits fine but has a little bit of play and moves forward a few millimeters

http://i67.tinypic.com/1pf5eb.jpg

Yeah you're going to want it in there before shooting the launcher. Should be locked up nice and tight. Just play around with it, try different sides, try flipping it around and try different sides again. I usually have to push the launcher down hard to get it to fit in place in the M4 barrel cut, place the butt stock on the ground. Then latch it and you're good. Washers are probably for the bolt, or screw, to mount the KAC latch to the M203 receiver.

The star is essentially 4 different spacers to mate up the launcher to the barrel.  Just flip it around till it fits tight.
I think the relieved semicircle goes away from the muzzle end, it's been a while.

No, the relieved semicircle faces the muzzle. The washers go under the 2 side screws which secure the M203. Use whichever "ninja star" cut is deepest first (facing forward) and attempt to mount the launcher. I think the unmarked one is deepest, then they get shallower with each dot.

When mounting, first locate the rear indexing pin @ the 6:00 slot of your barrel nut. Then ensure the rear of the trigger guard is on the magazine well front lip. You have to squeeze the launcher backwards a bit against the trigger guard. It's springy. You should be able to engage the launcher upwards at the front once you've done this (while holding rearward pressure). The star plate semicircle will slide up against the relief/stop notch of your barrel (if using M4 barrel). It should be tight. If it's not tight enough, remove the launcher and rotate your plate to try the second deepest slot, and so on, until you get a tight fit. Mine is tight using the deepest slot. I think most are, but they have 3 more to compensate for worn out stuff.

Once you've determined the appropriate plate slot, mount the launcher and secure the clamp assembly. If you've got the button looking retainer, it has to be rotated to the correct orientation AND pushed in while the latch goes down over it. Then you release pressure on it, and rotate it inboard. Your latch will not be accidently released. Ever. 

If you're mounting it to an M16 barrel, you'll need a KAC M5 rail and the M203 barrel stop, which only ships in the boxes of military NSN M5 rails. (Or equivalent stop appropriate to whatever brand rail you're using.) It has to perform the same function as the M4 carbine barrel notch and give a positive stop surface.

Barrel stop for M5 RAS:

Instructions are in TM 9-1010-221-10
Page 0030 00-1
(153 in the pdf)

Here's the first link I found for it:
http://www.liberatedmanuals.com/TM-9-1010-221-10.pdf

Good find. 
Note they depict the older wire retainer. Everything else works the same though.

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Thanks for correcting my bad advice.  I had originally posted the correct way and went back and second guessed myself.
Glad there are smarter people here than me