Do you have any tips for using old camo uniforms to camouflage your rifle?

You can tie it on the rifle, but it's just more efficient to put you and your rifle under a net, once you are done moving.
Even easier, on your boonie hat, the ghillie net going down the back should be long enough to cover the rifle when you turn it around after you get in position.

While I don’t have a tip on the AR, I must say I read your profile and found it very interesting.

Why?

You could use the cammies. If you want to break up the outline of your rifle, do it increments. Just remember you don't want it to snag on anything when you walk and don't go over board adding, less is sometimes the best.

In retrospect, after seeing the variety of comments, I guess we should have asked is if you wanted to "camouflage" (environment blend) your rifle with this method (like the helmet) or improve the aesthetics to make it look better. A case of functionality vs. "show"

Depending on your environment, and seasonal surroundings, will dictate your camouflage.

Moving through the bush, carrying your AR with it broad side will always show more of a firearm.
I've learned to keep my firearm pointed forward in movement. Anything, showing a smaller silhouette is better, as to looking at a rifle with it pointed towards you.

A black AR in a dark environment will not show well, but blend in your clothing and surroundings. So, you see, in snow, a white firearm is very hard to see.

Take a picture of you and your firearm, in what area your are moving in, look at it, and the picture will show you what you need to do, to camouflage your firearm.

Look at my picture in the jungles of Panama, see the sling, the barrel of the M-16's? Look at the guy with his back turned, see the slung M-16, and all the white faces?

Look at my picture with me sitting on the ground in the DMZ Korea, if it weren't for my white hankie, you wouldn't know that you were looking down the barrel of my M-2 50cal.
In the other picture, you can see behind my M-2, what it looks like, looking out.

Keep in mind, "Your camouflage is only as good as it's weak points", take the pictures, of yourself in your environment, you be the judge, and ask others what do they see, don't tell them what to look for.

As a sniper in a SWAT team for 7 years, the best way to hide your rifle, is to be hidden behind the environment, or under your clothing.
Also, as a sniper in a SWAT team, it taught me what to be looking for, as a sniper. Constance movement of a target, always got my attention, it taught me, not to be moving constantly, but a more stop, look around, then move again, to a short distance behind my environment.
It's not your firearm giving your location, it's your movement.

When I was carrying a AR on a mission in SWAT, an AR with a bull barrel, my assignment was to engage multi targets in a short time, and that was engaging at least 5 targets in 5 seconds, and I did just that.

I shoot old WWII era Finnish rifles, and have found that burlap worked great back then, and still works great now. Take a brush and give it a light brushing to make it "fuzzier" keep it off of metal surfaces that get really hot as it will still catch on fire. On old bolt action rifles it is not a problem, on a semi auto with "Iraqvet8888 unlimited ammo," anything is possible.