This is for the 590A1 with the heavy barrel. If you have the standard 590, it will be a lot easier and you probably won't have to do any fitting.
The handguard is the Mossberg stock handguard in park finish.
To start with I had to get two slightly smaller diameter and longer machine screws and nuts. Sorry, I don't remember exactly what size. I would have written it down if I had realized how many times I was going to have give out this info. Luckily they did have some already in tactical black. Make sure that that you don't get them too narrow, you want them to stay secure and not go TU on you. You also want them to be at least two and half to three inches long so you will have enough room to work with them and to make it easier to secure the nuts without having to try to apply a lot of leverage on the forward barrel ring.
To fit the guard on the barrel, you will need to rotate it slightly to the side (9 o'clock or 3 o'clock) and hold it parallel to the barrel. Slip the rear barrel ring over the barrel (you want to do this at a point where the rear barrel ring can slip over the narrower barrel diameter as it tapers towards the muzzle) and start sliding the guard to the rear toward it's final position. You might have to rotate the barrel back towards its normal orientation and position (top dead center) on the barrel as you slide it to the rear. The forward barrel band should be slipped over the barrel as it clears the front sight assembly. You may want to put some masking tape on the barrel to prevent scratches for this step as it might be a tight fit depending on your guard and barrel.
You will have to relieve some of the metal on the underside of the forward barrel band. This is the part with the two "ears" that come down and that the cross screws secure. This piece is spot-welded to the actual body of the shield, so be careful that you don't remove too much metal. You will have to test fit the guard as you go to see how much you need to take off.
You will also have to slightly extend/enlarge the cross-screw holes in the ears to give the cross-screws enough clearance below the barrel. I'm not talking about much, just a little.
You will have to do several trial fits to make sure that you don't take to much metal off of the guard. As you are fitting it, check the clearance of the cross-screw holes when the barrel is mounted to the receiver and with the handguard in its proper position. You will need to fit the screws through the forward barrel band to make sure that you can get the nuts on them. Do not trim the length of your cross screws yet.
Once you have the guard modified so that you can get the cross screws through the forward barrel ring, take it off. Apply some cold blue, Casey's touch up, Duracoat, or whatever. You just want to apply some protection to the exposed raw metal so that you don't get any corrosion under the guard.
Trim the plastic spacer that comes with the guard as directed in the instructions. As always, trim slightly oversized so that you can make a final fit to test for proper length and fitment of the spacer. It may be necessary to remove some material from the top or bottom surface of the spacer again depending on your barrel and guard.
Once you have the guard like you want it, install the guard and secure it with the cross screws. Now you can trim the length of the screws, and touch up the raw metal ends.
You now have the rare honor of what Mossy says you can't do and I've been doing for about seven years.