Step 1: Materials.
Some strong string or glue.
Camouflage netting/material (It must match the terrain/area and match your ghillie suit.)
Mesh (like scrim scarf material)
A rifle (duh)
Some spare time.
Step 2: Construction.
Mesh to the appropriate color.
Okay, it’s a good idea for your rifle to have a camouflage base coat (painted) but if that’s not possible it doesn't really matter that much. First you should cut up the material to the desired length, then cover the main body of the rifle (where the bolt/trigger/scope etc is) with the camo netting. Make sure to mount it securely to say the scope rings and other anchor points. However make sure that you have left space for the trigger, magazine etc. The butt of the rifle should be the next priority, so put some netting/material around the butt area. Try and make it so the material doesn't move and that when you raise the rifle to your cheek its comfortable. If it isn't take it off and put some warm and smooth material around the butt, then add the netting around it. (cut out a hole in the netting for your cheek.) At this point check if everything is secure and there aren't any uncovered places that you've missed (in the main body and butt) that need to be covered, fix them up. Now we move onto the barrel. Cover it all the way around with your netting, then add a small circle of dyed hessian (match it to the diameter and shape of the end of the barrel) and attach it only on the top. The idea is that the speed of the bullet forces air out of the barrel very quickly and that air pushes the hessian out of the way (making it go up) before the barrel exits the barrel. This makes for good camouflage as any shine could lead to your detection. Again, fix up uncovered places. Now its time for the accessories, by this I mean bipods and other stuff like magazines. For the bipod, just wrap the netting around it and secure. This is generally the same for magazines but attach the netting while it’s in the rifle. Now, another important bit, the end of the scope, the part of the scope that you look through should go uncovered but for the other get your dyed mesh and attach it to the top of scope as close as possible to the opposite end of the scope as that of which you look through. Now pull it tight over the bottom of the same part of the scope and secure.